Tony Blair has defended his decision to strengthen the Downing Street machine, but conceded that mistakes were made in the use of spin during his first term.

He told the liaison committee that in the modern world it was essential to ensure a prime minister's writ ran throughout government, saying that in the end the bucked stopped with him.

But he denied that his approach was undermining the traditional form of Cabinet government - insisting it was "alive and well" under his leadership.

Sir George Young, a former Conservative minister, suggested that the traditional Whitehall machine had been "short-circuited" by the prime minister's style of centralised government.

Blair said such claims were "unfair and wrong".

While he had "regular bi-lateral stocktakes with ministers", it was departments that were charged with policy.

"The reality is for any modern prime minister you also want to know what is happening in your own government, to be trying to drive forward the agenda of change on which you were elected," he said,

"It is true that at Cabinet, yes, I would be surprised if the first time I knew of problem is if it suddenly surfaced round the Cabinet table. But I regard that as good management in a sense."

Such a surprise would indicate that communications between departments and the centre had broken down, he argued.

"I do probably place a lot more emphasis on bi-lateral stocktakes," he added.

He said all prime ministers were accused either of being dictatorial or weak. "You pays your money and you takes your choice really," he said.

Blair said he would "make no apology for having a strong centre".

"I think you need a strong centre, particularly in circumstances where, one, the focus of this government is on delivering better public services....and the second thing is that in relation to foreign policy and security issues the simple fact of the matter is that in today's world there just is a lot more that needs to be done at a prime ministerial level," he said.

The prime minister said his Number 10 fiefdom had fewer people than the Irish equivalent, or than his opposite numbers in France of Germany.

He said some extra staff were needed to deal with the 500,000 letters received in the first seven months of this year - while in terms of foreign policy it "just isn't possible" for one person to do the job.

"In relation to policy, yes, again its true, we've expanded the number of policy advisers, but that's because I think its the right thing to do," he said.

"I think its important that in these big public service reform areas we are in constant dialogue, keeping up an exchange of views and partnership with the departments to drive forward the process of change."

But he said the changes did not mean he was running his own prime minister's department and denied that Cabinet government had been weakened.

"I think you need to get this in context, strengthening the centre, yes...I can't believe there is a single prime minister, and some of you have experience of them round this table, who hasn't wanted the prime minister's writ to run.

"I can't believe there is a prime minister who is sitting in Downing Street saying, 'Let them just get on with it, I don't mind much'. Its not the real world. 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 added: "I think the process of Cabinet government is alive and well, I have to say, I don't think its inconsistent with a stronger centre."

On special advisers, he disagreed with Labour chairman Charles Clarke who has in the past suggested there are too many now employed in government.

"I think obviously we have the right number otherwise we'd have fewer of them," Blair said.

"In respect of special advisers there's often talk of them being media people. Actually few of them are media people, the vast bulk of them are policy people."

But Blair conceded that Labour's spin machine had failed to realise the differences between being in opposition and being in government. Asked if the government had spun too much, Blair said he now wanted to do things "differently".

"When you are in opposition for 18 years as we were, there was a tendency, because this is the way that opposition works, that you believe the announcement is the reality," he said.

"And in many ways it is, because what matters is the policy you are announcing. You are not actually in a position to deliver anything on the ground.

"I think for the first period of time in government there was a tendency to believe that the same situation still applied. It doesn't in fact.

"For government the announcement is merely the intention, the reality is what you have to go and deliver on the ground.

"I think that for us, in a sense, doing it this way, making sure we have more ministerial statements, making sure we try and find new ways of reaching out in direct conversation with people, is a way of overcoming what is perceived, I think often unfairly, as issues to do simply with news management."