Cuts to tax credits will not be reviewed in the Autumn Statement, David Cameron has insisted amid calls from his own party to think again.

The Prime Minister said he felt the package was the right way to reach the ambition of raising wages and cutting taxes to offset big cuts to in-work benefits.

Critics have condemned the plans, claiming the measures will leave millions of working families out of pocket. The Institute for Fiscal Studies has warned it is "arithmetically impossible" for nobody to lose out under the changes.

Former Conservative minister David Willetts urged Mr Cameron and Chancellor George Osborne to look at the plans again as the Tory Party conference got under way in Manchester.

But speaking to BBC One's The Andrew Marr Show, Mr Cameron said the introduction of the national living wage and continued increases to the personal tax allowance would protect the poorest.

He said: "We have had the vote in Parliament on tax credits and I think people respect this argument that the national living wage - a 50p increase next year, so a £20-a-week pay rise, rising to £9 by the end of this Parliament - that is a very significant change that really helps to make work pay rather than a tax credit system that recycles money back to people.

"Obviously I accept we are making changes to tax credits. We are protecting the lowest-paid people with child tax credits and what goes with it but we are moving to an economy where you get paid more and where you pay less in tax, rather than paying more in tax and getting the money back in tax credits. That is a better system.

"As a country, we have had to make difficult decisions in order to get rid of what was the biggest budget deficit almost anywhere in the world. Of course, if you don't tackle excessive welfare and make reductions there you have to either put up people's taxes or cut the NHS or cut education, which I don't want to do."

Asked directly about a further review ahead of next month's Autumn Statement, Mr Cameron said: "No, we think the changes we have put forward are right and they come with higher pay and lower taxes."

Mr Willetts told The Times ahead of the conference: "There is a real risk that it could turn sour as some of those hard-working families that politicians love realise they are heavy losers."

He added: "If the goal is a genuine blue-collar conservatism, it must be a priority for the autumn statement and spending review to ease a policy which could otherwise do the same kind of political damage as Labour's abolition of the 10 per cent income tax band."

The Conservative Party conference will get under way amid a mass protest in Manchester, with up to 70,000 people expected.

But Mr Cameron told the programme: "We are making changes to tax credits and there is an important big picture here.

"If you look at the tax credit system, it wasn't working. When I became Prime Minister, nine out of 10 families were getting tax credits, including Members of Parliament. So it was helping families right up the income scale.

"One more figure - if you look at the last 12 years before I became Prime Minister, the tax credit system went from costing £6 billion to costing almost £30 billion but in-work poverty actually went up. It didn't work."

Asked about the IFS analysis, which says the increase to the minimum wage will not make up all of the shortfall in tax credit cuts, Mr Cameron said: "That's not right. If you take a family where someone is on minimum wage, when you take into account all the things we are changing in tax, in the national living wage and tax credits, that family will be better off - not least because you have got the national living wage and we are also cutting tax, so you can earn £11,000 before you start paying any tax at all."

Mr Cameron said that he did not plan to take the Conservatives to the right in response to Labour's expected shift to the left under the leadership of Jeremy Corbyn.

He insisted that there would be "no complacency" at the Manchester conference about the party securing a majority in the Commons amid disarray among its main rivals.

The Prime Minister told Marr: "We will be absolutely anchored in the common ground of British politics. We were given an instruction by the British people to deliver security at every stage in your life - securing good jobs, apprenticeships, cutting people's taxes, making sure there was childcare there. We are going to deliver on these things.

"As others are losing their heads and lurching off, we will be absolutely in the common ground, delivering for the working people of Britain."

Asked whether he now had an opportunity to "kill off the Labour Party as a serious national force", Mr Cameron said: "We are not in control of what happens to the Labour Party. It seems they have - as I put it - run off to the hills. We are in control of what we do.

"There'll be no complacency or back-slapping here this week - maybe a little bit of mild celebration of the election. The main theme is going to be about delivering people the things that we promised."

The general secretary of the Unison union, Dave Prentis, said: "Millions of low to middle income working families will be the victims of the Chancellor's cruel tax credits snatch-and-grab next year.

"Ministers are punishing millions of working people who just want to provide for their children. The Government must think again, admit tax credit cuts were a huge mistake, before millions of families suffer yet more pain under austerity."