GORDON Brown has warned Theresa May could be replaced by a new Tory Prime Minister if she loses a crunch vote on Brexit this week.

The former Labour PM said the current occupant of Number 10 could be turfed out by defeat over the EU Withdrawal Bill, which returns to the Commons on Tuesday and Wednesday.

However Mr Brown said the fixed-term parliament act would make it possible for the Tories to stay in office, rather than face a general election, after Mrs May was replaced.

The government has accepted just one of 15 amendments made to the Withdrawal Bill in the House of Lords, and is now fighting to reverse or water-down the rest.

Around a dozen pro-EU Tory rebels voting with Labour, the LibDems and SNP could inflict defeats on key amendments on the customs union and a meaningful vote on Brexit.

The first would commit the government to informing parliament of steps taken to negotiate a customs union, while the latter would empower parliament to reject the final Brexit package and send the government back to the EU negotiating table.

There is also a vote on whether the UK should stay in the single market via membership the European Economic Area (EEA) after leaving the EU.

Research highlighted by the Open Britain group said staying in the customs union but not the EEA - which is Labour’s position - could cost Scotland’s economy £2.2bn a year by 2030.

Asked what would happen if the Government lost the customs union vote, Mr Brown told BBC One's Andrew Marr Show: “There is potentially the chance of a change of Prime Minister, but I don't think at that point they will want to give up as a Government, and I think the five-year parliament makes it possible for them to survive.”

He said that if parliament rejected the final Brexit deal in the autumn it would be a “point of further crisis”.

Reflecting on his own political shelf-life, he said Jeremy Corbyn was a “phenomenon”, but added: “Jeremy himself is going to accept that nobody goes on forever, we're all phases.”

Mr Brown’s warning came after The Observer reported Tory rebels were backing away from defeating Mrs May in case Boris Johnson became PM instead.

One former minister told the paper: “If we were to defeat her on that now, does that further weaken her and give [Jacob Rees-Mogg’s] European Research Group more opportunities to stick the knife into her? That is not where we want to be.”

The Remain-supporting former Home Secretary Amber Rudd also urged rebels to unite in support for Mrs May, saying backing the government was a “no brainer”.

In a joint newspaper article with arch-Brexiter Iain Duncan Smith, Ms Rudd said Jeremy Corbyn would exploit any setback and try “to frustrate Brexit for his own political ends”.

The two wrote: “It behoves us all to demonstrate discipline and unity of purpose in support of the Prime Minister. We cannot allow ourselves to become divided and risk losing the precious chance to go on implementing policies that transform lives."

Former deputy PM Damian green predicted most of the key votes would “pass easily”.

Shadow Brexit secretary Sir Keir Starmer said pro-EU Tory MPs could alter the direction of the Brexit negotiations by voting with the opposition on the key amendments.

He told the BBC: “There is a real chance for Parliament to bring some order where there is real chaos.”

Former chancellor Ken Clarke said the “meaningful vote” amendment would strengthen Mrs May's hand against Brexit Secretary David Davis and Mr Johnson.

“What we need to do is to rescue the Prime Minister from this terrible treatment she is getting from key members of her Cabinet,” he told the BBC1 Sunday Politics programme.

“We can give Parliament the opportunity of rescuing Theresa from all this and begin to use the parliamentary majority in favour of a softer, sensible Brexit."