Home Secretary Sajid Javid has said the Government will support an amendment demanding a treaty on citizens' rights after Brexit, despite Theresa May saying it is not something the EU has the authority to do.

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The Prime Minister was expected to go down to defeat in the Commons on Wednesday on the amendment from Tory MP Alberto Costa, after Jeremy Corbyn announced that Labour would support it.

Mr Costa's office was unable to confirm or deny reports that the South Leicestershire MP was standing down as a parliamentary aide to Scotland Secretary David Mundell in order to push his plan.

Mr Javid appeared to be caught by surprise while answering questions before a parliamentary committee to hear that his support for the move was not shared by Mrs May.

He told the Home Affairs Select Committee there was "nothing" wrong Mr Costa's proposals.

Asked by SNP committee member Stuart McDonald whether the Government was now supporting them, he replied: "Yes, what do you mean now? When was the Government not supporting it? When did you hear that?"

When Mr McDonald said "Yesterday", Mr Javid asked: "From who?"

The SNP MP replied: "The Prime Minister."

A visibly nonplussed Home Secretary replied: "Did you? Right... Oh... OK. I'm perfectly happy with the amendment.

"What Mr Costa is doing, which I think is perfectly correct, is trying to find more ways for Parliament to give that reassurance."

More than 50 Conservatives are among the 141 MPs who have signed the amendment, which calls for a separate agreement with the European Union to protect the rights of expats even if there is a no-deal Brexit.

Among those signing are former leader Iain Duncan Smith, European Research Group chair Jacob Rees-Mogg and leading pro-EU Tories like Dominic Grieve.

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After Labour announced it was supporting the amendment, Mr Costa said it would be a "farce" if the Government did not back down.

He told the Press Association on Tuesday: "If the Government weren't to accept this tomorrow, they will be staring an enormous defeat in the face."

And he acknowledged that his job as a parliamentary private secretary (PPS) was on the line as a result of his actions.

"I have considered my position and I have tabled the amendment," he said then. "It is entirely in the hands of Government to decide who is and is not a PPS. I will continue in my role until told otherwise."

In the Commons on Tuesday, Mrs May acknowledged concerns about citizens' rights if her deal did not go through.

She said that the Government had guaranteed "the rights of the three million EU citizens living in the UK will be protected".

"But a separate agreement for citizens' rights is something the EU have been clear they do not have the legal authority for," she said.

"If it is not done in a Withdrawal Agreement, these issues become a matter for member states unless the EU were to agree a new mandate to take this forward."

Foreign Secretary Jeremy Hunt was seeking assurances from counterparts in EU states on the issue, she said, adding: "I urge all EU countries to make this guarantee and end the uncertainty for these citizens."