Boris Johnson has issued a stern warning over welfare reform saying workers "who get up in the small hours or work through the night" must be protected.

In his last speech as London Mayor to the Conservative Party Conference, Mr Johnson said the Conservatives could not ignore the "gulf" between the pay packets of the rich and the poor.

And in what is being seen as an attack on the cuts to tax credits that will hit millions of people, Mr Johnson said it was vital that the aspirations of "retail workers and cleaners" were safeguarded.

He said firms must pay their due in taxes and must pay a proper living wage to workers but also stressed that "we must ensure that as we reform welfare and we cut taxes that we protect the hardest working and lowest paid.

"The retail staff, the cleaners, who get up in the small hours or work through the night because they have dreams for what their families can achieve.

"The people without whom the London economy would simply collapse. The aspiring, striving, working people that Labour is leaving behind."

Mr Johnson, a front-runner in the race to succeed David Cameron, pointed out that in 1980 a chief executive of a FTSE 100 company earned about 25 times the average pay of their employees but today that was 130 times.

His speech comes after Mr Cameron, in an interview with Sky News, was forced to defend tax credit cuts, which will see three million people notified just before Christmas that they will lose £1,000 a year from April.

The Prime Minister insisted that people would be £20 a week better off because of the introduction of the new National Living Wage and the rise in the level at which people start to pay income tax.

Mr Cameron said the Government was moving the UK away from a "low-pay, high welfare and high tax" society to one with "lower taxes and lower welfare."

He said: "Now I accept that we are combining those two measures (National Living Wage and income tax) with a reduction in welfare, not least because we need to get on top of our national finance, get our budget deficit down and have welfare and public services in our country that we can afford for the long term."

Mr Cameron added: "The best way to help people on low pay is to put up their pay and cut their taxes.

"I think the old system where what we did was we took away money from people in taxes, gave it back to them in tax credits created an expensive and unworkable merry-go-round whereas what we have now is you earn more, you pay less in tax and I think that makes for a more efficient economy."

The Institute for Fiscal Studies has warned that it was "arithmetically impossible" for no one to be left out of pocket and claim 13 million families will lose an average of £240 a year.

Mr Cameron was also forced to defend Health Secretary Jeremy Hunt over comments he made on Monday which implied that tax credit cuts would make sure British people work as hard as the Chinese to help the country compete on the world stage.

Mr Cameron said Mr Hunt had been "widely misquoted" and that the Health Secretary had subsequently come out and said British people worked hard.

He also defended the Government treatment of the hundreds of steelworkers who have lost their jobs in Redcar, saying: "we will do everything we can to help".

He said £80m had already been made available for training and regeneration.