David Cameron has insisted that the Government's plans to cut tax credits for poorer families are "fair", after Boris Johnson became the latest senior Conservative to warn about their impact on the lowest paid.
The London Mayor is expected to tell the Conservative Party conference in Manchester that when the welfare cuts come into effect in April, the Government must ensure it protects people like shop workers and cleaners on whom the capital's economy depends.
The Prime Minister is coming under pressure to rethink the plans, amid predictions that millions of working households will be left out of pocket as tax credits are slashed ahead of planned rises in the minimum wage and further cuts to the personal allowance.
An estimated 60,000 marched in protest against austerity on Sunday, and there was an outraged reaction to suggestions from Health Secretary Jeremy Hunt that cuts in in-work benefits would send a "cultural signal" to Britons to work like the Americans or Chinese.
But Mr Cameron insisted that eight out of 10 families will be better off overall as a result of the Government's welfare and tax changes. Even those who lost out in cash terms would gain from measures like extended free childcare and reductions in social rents, he said.
The Prime Minister told BBC1's Breakfast: "The combination of tax reduction, a higher national living wage and these tax changes, I believe, is fair. They are part of moving the country form a low-pay, high-tax, high-welfare country towards a higher-pay, lower-tax, lower-welfare country."
Mr Cameron also backed Home Secretary Theresa May's warning that mass migration will make it "impossible to build a cohesive society".
Mrs May is expected to use her keynote speech to the conference to warn that millions of people from poorer nations have a "perfectly understandable" desire to live in Britain but there is a limit to the amount of migrants the country can take.
"While we must fulfil our moral duty to help people in desperate need, we must also have an immigration system that allows us to control who comes to our country," the Home Secretary will say.
"Because, when immigration is too high, when the pace of change is too fast, it's impossible to build a cohesive society. It's difficult for schools and hospitals and core infrastructure like housing and transport to cope. And we know that for people in low-paid jobs, wages are forced down even further while some people are forced out of work altogether."
Mr Cameron rejected suggestions that Mrs May - whom he hailed as an "excellent" Home Secretary - was seeking to say that unemployment was the fault of foreigners
He told the BBC: "No, what we are saying is that in an open, modern democracy like Britain, yes, we need migration, we benefit from migration, but that migration must be controlled.
"One of the problems we have had in recent years is, because we have created more jobs than the rest of the EU put together, we have seen very high rates of migration."
Mr Cameron said the Government needed to address a situation where new migrants could claim as much as £10,000 a year in in-work benefits as soon as they arrive in the country.
"It's quite clear that we need to do more to bring migration into better balance because, as Theresa will say in her speech, if you want an integrated, successful society, yes, you want immigration, but you want it at a rate where you can properly integrate people and bring them into your society and make sure that the school places are there and the hospitals aren't overcrowded and the pressure on our country isn't too great."