The prime minister has defended his reorganisation of Downing Street and insisted that the public service reform programme will deliver improvements.

Relaxed and shirtsleeved, the prime minister kicked off what proved to be a marathon two-and-a-half hour session before a "star chamber " of select committee chairmen just after 10.30am.

He faced questioning on four themes: his role and relationship with parliament; the delivery of public services; international affairs and the war against terror; and the quality of public life.

Whilst the prime minister was not made aware of the questions before entering the Portcullis House committee room, he was given advanced notice of the subject areas.

In his opening statement Blair said he was pleased to be the first prime minister to submit himself to scrutiny by a select committee.

"It has been the custom and practice over many years for prime ministers to refuse to come before select committees, and I'm happy to reverse that practice today - at least I am at this stage," he said.

The prime minister said responsibility for the government's performance rested with him. "The real world is the prime minister, in the end the buck stops with you. That's the top job, that's how it should be," he said.

First to confront him was the Labour chairman of the public administration committee, Dr Tony Wright.

He asked the Labour leader why he had finally decided to submit himself to questioning when he had refused on countless occasions before.

Blair, initially looking nervous under direct questioning, said his decision was part of "doing things differently".

It was, he said, "my desire to engage in the policy debate in a different way".

Under further interrogation, he conceded that the weekly session of prime minister's questions was "80 per cent theatre" and accepted that the spin techniques of opposition were not necessarily the right ones for government.

"Prime minister's question time is an excellent way of holding the prime minister to account but if we are all absolutely honest about it, it is 80 per cent theatre as well," he said.

Sir George Young, the former Tory cabinet minister, quizzed Blair on the radical changes he had made to the structure of Number 10.

Blair said it was important to have an office which can be relied upon to "do the job effectively".

"I make no apologies for having a strong centre," he said.

But he stressed that his Number 10 team was smaller than that of the Irish prime minister and considerably lower in comparison to his French counterpart.

But Dr Wright demanded to know why Blair was "so coy" about admitting that he had ripped up the old system and imposed a more centralised approach to government.

Blair was again forced to defend the explosion in the number of special advisers and spin doctors.

"I am not disputing the fact that we have strengthened the centre considerably but I say that is the right thing to do," said Blair.

"I can't believe there is a single prime minister who hasn't wanted the prime minister's writ to run."

And he suggested that future occupants of Number 10 would follow his example.

"I have a kind of hunch that most prime ministers will want to keep that strengthened centre," he said.

After a second intervention from Sir George, Blair insisted that special advisers played an important part in supporting government - but said it would be "absurd" to suggest they were in the driving seat.

"Special advisers have a role to play and often that role is misunderstood," he said.

"But the idea that they determine the policy of government would not be recognised by one cabinet member."

The PM also defended the plethora of new units set up in Downing Street under his stewardship.

He insisted that they were aiding a cross-department flow of information - rather than taking policy decisions over and above ministers' heads.

"I simply don't recognise this notion that policy is not made in departments," he said.

"In the end ministers are accountable and it should be ministers that are held to account...There is no minister in my cabinet who is not the best person to explain what that policy is," said the prime minister.

Edward Leigh, the chairman of the powerful public accounts committee, took Blair to task over the Office of the e-Envoy, which employs 270 staff at a cost of over £35 million a year.

Leigh asked why the unit could not say whether it would meet its key targets.

Blair said the e-envoy's office reported regularly to ministers and insisted that it was on track to meet its targets for the electronic delivery of public services by 2005.

He was also forced to defend the government's actions in relation to parliament.

Sir Nicholas Winterton, the outspoken Tory MP, said it was "deeply worrying" to the public that parliament was being sidelined.

Blair refuted claims that he was riding roughshod over the Commons, stressing it was important to get journalists to report on the "detailed proper scrutiny" which takes place on the floor of the House.

"If what people see is a political discourse which takes place solely in terms of process and personality and not policy I think they do become disconnected over time," he said.

The prime minister admitted that there was a "great deal of pressure to get legislation through" but said that there should be more pre-legislative scrutiny on technical legislation.

Quizzed on the future of the lobby system, which sees a select band of Westminster journalists being given a twice-daily Downing Street briefing, Blair did not rule out ending the closed shop nature of the system.

He said the briefings would continue, but stressed "how we change that over time and make it more open to other journalists too...is an issue".

As the debate moved onto the delivery of public services, Blair dismissed suggestions that the chancellor's radical reform and investment programme was premised on unsustainable growth forecasts.

The prime minister said the chancellor's forecasts were based on "cautious assumptions".

"The record of the last five years is that these assumptions are well made," said Blair.

But he conceded that the government must be careful to manage the public's expectations about what can be delivered.

"The problem is that if you announce extra money for the health service people turn up at a doctor's surgery the very next day and ask 'where is it?'."

The prime minister said it was important to stress that whilst a "step change" was taking place, improvements could only be delivered "bit by bit, by bit".

Questioned on why additional investment in healthcare appeared to be delivering little benefit for patients, Blair said the NHS was in "better shape than 1997" but conceded that it was still suffering from a capacity problem.

"There are real improvements but there is a long way to go," he said.

Put on the spot over transport by the formidable Labour MP Gwyneth Dunwoody, Blair was forced to defend his decision to draft in blue skies thinker Lord Birt.

He said the Department of Transport was still in the driving seat but admitted that he held "regular stock takes" with the ministry to ensure new policies were being examined.

"There is a huge transport challenge - it is the area where you need people coming forward with reform ideas," the prime minister told the committee.

"Transport is one area for me where blue skies thinking has never been more appropriate."

Lib Dem MP Archie Kirkwood, who chairs the work and pensions committee, expressed concern that the chancellor was effectively micro-managing Andrew Smith's welfare department.

Kirkwood claimed that the Treasury was now involved in "policy formulation" and challenged Blair to accept the blame for the change.

Blair said there was a great deal of "mythology" surrounding Downing Street and the Treasury's role in policy matters.

"There is a discussion between us all but the actual policy detail is worked out by them," he said.

As the debate shifted to foreign affairs, Blair insisted that had not done any deal with the US on strikes against Iraq.

"There are no decisions which have been taken on military action," said Blair.

But he signalled that pre-emptive action could be more effective - particularly where weapons of mass destruction were involved.

"It is clear that Saddam Hussein is still trying to develop weapons of mass destruction... over time you are entitled to conclude that this threat is growing and not diminishing," said the prime minister.

"If there is a gathering threat of danger let us deal with it before it happens. There is a threat, the threat has changed since September 11. We do have to deal with it, but how we deal with it is still a matter for discussion."

On Northern Ireland, Blair said the peace process was about more than a temporary ceasefire.

He told MPs it was about moving towards sustained democratic politics - stressing the Good Friday Agreement was a "process and not an event".

But he conceded that it would "take time to squeeze every bit of sectarian violence out of [Northern Ireland]".

On the euro, Blair told MPs that he was standing by the five tests for entry - insisting the "position has not changed".

He also rejected suggestions that the government has ruled out a euro poll during the course of this parliament.

Whilst the committee gave Blair a round of applause at the end of his first twice-yearly question and answer session, MPs accused them of missing a trick.

Graham Allen, a former Labour whip, said the event had turned out to be "a gentle photo op-for the PM and MPs".