The prime minister has taken to the nation's television screens to deny claims that he broke election promises as Downing Street's post budget "charm offensive" continues.

Appearing on the Frost programme, Tony Blair dismissed accusations that Labour misled voters over tax rises that would inevitably be needed to fund its planned programme of public service investment.

National Insurance increases had never been positively ruled out, he insisted.

He said the tax was not income tax and was not covered by the 2001 manifesto promise "not to raise the basic or top rate of income tax".

"[National Insurance] is not the same as the standard or top rate of income tax where we made our commitments," he said.

"It doesn't apply to pensioners, it doesn't apply to savings. All things come out of income but during the election campaign people were constantly pressing us to give assurances on National Insurance. And we said 'look we're not writing the budget now," he told the BBC.

The prime minister used a similar formula when challenged over whether further increases could be needed to fund public services.

"I am not going to sit down and write the budget. I am just going to say what I have always said on these things. I can't go and write the budget. What I can say is the changes that we made in the health service are funded by what we did on Wednesday," he said.

"No one ever likes paying tax but even after these changes the tax burden under this government will be lower than in many of the Thatcher years, lower than the European average, lower than any of our major, comparable European competitors."

Blair highlighted Labour's promises and successes on the economy, unemployment education.

Confirming newspaper reports that an NHS spending boost had been on the government's agenda for two years, the PM said: "We promised and I repeated that promise here on this programme some years ago, that we would get extra reform and investment into the NHS."

Buoyed by polls showing backing for Gordon Brown's NHS tax increases, Blair rejected accusations that he was returning to an old-Labour "tax and spend" strategy.

He said the "vast bulk" of middle class voters backed the plans to put more money into the NHS.

In a move that may dismay powerful public sector unions and Labour backers, the prime minister compared the task ahead to the union-busting industrial reforms of the 1980s.

"There is a challenge. It is a big thing. It is as big as the industrial restructuring in the 1980s was for the Thatcher government. And it requires very difficult choices," he said.

"It is a huge enterprise. There is no point being in politics unless you take the big challenges on. If it fails, I will carry the can but I believe it won't."