LABOUR MPs have reacted with fury after Jeremy Corbyn offered to help Theresa May get a Brexit deal through the Commons.

In a letter to the Prime Minister on Wednesday night, the leader of the opposition said his party would support the Government if May rewrote the political declaration she’s agreed with the EU to include, among other things, a customs union.

At least one Labour MP has said he’s considering quitting the party.

The political declaration sketches out a future relationship between the UK and the EU, and though already agreed by negotiators in London and Brussels, it’s not as difficult to unpick as the withdrawal agreement which contains the controversial Irish backstop.

Corbyn has made five demands of the Prime Minister, firstly a permanent customs union; close alignment with the single market and a say in trade deals; alignment on “rights and protections”; participation in EU agencies and funding

programmes; and “unambiguous agreements on the detail of future security arrangements”.

READ MORE: May leaves EU empty-handed after talks fail to bring about change to backstop

The letter came a day after a leaked report drawn up by the Labour-affiliated transport union TSSA revealed that the party could lose over 40 seats – including five in Scotland – in a snap General Election unless it shifts away from its pro-Brexit stance.

Owen Smith, who stood against Corbyn for the leadership, said he could now quit Labour.

“I think it’s something that I and lots of other people are considering right now,” he told BBC Radio 5 Live.

“At the moment I may be asked by the Labour Party to row in behind a policy decision that they know, and the Government knows, is going to make the people I represent poorer and – more fundamentally actually – is at odds with the internationalist, social democratic values I believe in.”

Prominent pro-EU MP Chuka Umunna said the position was “totally demoralising”.

He said: “This is not opposition, it is the facilitation of a deal which will make this country poorer.”

“I hate to think what all those young voters who flocked to the party for the first time in 2017 will make of this. Vote Labour, get a Tory Brexit. They will feel they have been sold down the river.”

Shadow international trade secretary Barry Gardiner defended the shift in policy. He said the offer was made “in a spirit of co-operation and compromise”.

“What we are doing is saying we believe that these are the options that are available that would actually secure a majority in the House of Commons,” he told the BBC’s Today programme.

Speaking later, David Lidington, the de-facto deputy prime minister, said he wanted to “understand exactly where the Labour frontbench is coming from”.

He said: “It doesn’t mean we will necessarily come to an agreement, but we need to take this forward … let’s hope those conversations can take place.”

The SNP’s leader at Westminster Ian Blackford it was “disingenuous” for the leader of the opposition to try and pretend this was “anything other than support for a Tory-driven, economy-wrecking Brexit deal”.

He added: “Labour under Jeremy Corbyn has shifted from being the official opposition to the official facilitator for a Tory hard Brexit that will hit jobs and living standards across the UK.”

There are now just 49 days until Britain leaves the EU.