Iain Duncan Smith has called on the Prime Minister to back £2 billion extra funding for welfare in this month's budget.

The architect of the troubled Universal Credit system quit in 2015 over billions being cut from the new benefits scheme.

Now he has appealed to the Prime Minister to back an extra £2 billion funding to get levels back to the original amount and rescue the system.

Mr Duncan Smith told Sky News's Sophy Ridge on Sunday that topping up funding was the best way of reaching the "just about managing" Mrs May promised she would help.

He said: "Theresa May stood on the steps of Downing Street and said 'I want to look after those who are just about managing'.

"Universal Credit is the single best system to get to those who are just about managing.

"It's better than the tax system, it's better than charitable giveaways - it gets the money to that bottom four deciles in a way no other system does."

The Conservative MP said the Chancellor needed to prioritise welfare payments over tax cuts if he wanted to help the poorest.

"The key thing is, the structure works, but we need to put the money in," he said.

"I think this Government is listening, I think the Chancellor is listening, and I'm asking him to do that.

"If we do that and get the money back to where it should be the reality is nobody should lose at all."

But Mr Duncan Smith defended the controversial decision to pay Universal Credits one month in arrears, saying it would identify those who could not cope.

He said: "If you are not capable of coping with that then you're going to go into a job and crash out quite quickly.

"Universal Credit will identify that minority of people who will not be able to move into work with those problems and then try and resolve them."

Shadow attorney general Baroness Chakrabarti said the UC name was now a "toxic brand" and it would be "very hard to rebuild trust in it".

She told Sky News's Sophy Ridge on Sunday: "The theoretical idea that you can have one simple benefit rather than people having to apply for lots of benefits always appealed to me in theory.

"The problem with Universal Credit now is, under this Government, is it is being used to cut people's benefits - to penalise people for having too many children, to penalise the disabled and other vulnerable people.

"The notion of a simpler benefits system - great idea, but not if you are adding austerity to it."

Speaking on the BBC's Andrew Marr Show, shadow foreign secretary Emily Thornberry called for a root and branch review of Universal Credit, which she said was "unfair".

Ms Thornberry said former chancellor George Osborne had used it as "a vehicle for cuts because it's so complex you could introduce a whole load of cuts and nobody would notice".

She added: "We must not come out of austerity on the backs of the poor.

"This is wrong, it's fundamentally wrong."

Health Secretary Matt Hancock defended the system, telling the Andrew Marr Show: "I have seen the Universal Credit in operation in my own constituency, where I have had no letters on it at all, whereas you normally get a pretty good feel for how much of a problem there is.

"Also I have sat there with the work coaches meeting people who are on Universal Credit and seen how the way that it works means that people can be helped into work much more."