Jeremy Corbyn is facing deepening divisions on his back benches after a group of MPs from Leave-supporting constituencies wrote to the Labour leader warning him not to soften his stance on Brexit.

In a letter sent ahead of crunch votes on Brexit in the Commons tomorrow, the MPs told Mr Corbyn not to “ignore” public concerns about immigration and claimed that continued continued close alignment with the EU, as championed by others in the party, would result in a “huge democratic deficit”.

The letter is a direct response to prominent Labour MPs, including Chuka Umunna and Chris Leslie, publicly demanding Mr Corbyn support a softer Brexit that would see the UK stay in the single market.

The Labour leadership is preparing for a major rebellion by pro-EU MPs, who will defy party instructions in order to vote for a motion that would in effect keep Britain in the single market by joining the European Economic Area (EEA). Mr Corbyn has ordered his MPs to abstain on the matter.

That decision prompted fury among pro-Europe MPs in his party, at least 70 of whom are understood to be ready to defy their leader in order to vote for the amendment.

The Independent understands several ministers and frontbench aides are among the potential rebels, raising the prospect of them either resigning or being sacked.

In a letter seen by The Independent, the MPs from pro-Brexit constituencies tell Mr Corbyn not to budge on his commitment to leaving the European Economic Area.

“We appreciate the significant efforts that you and Keir Starmer, along with his shadow DExEU team, have undertaken to ensure that Labour’s policy reflects our manifesto commitment to accessing the benefits of the single market, whilst acknowledging that freedom of movement must end when we leave the European Union,” they write.

“However, we are aware that some Labour MPs believe the Labour Party should back Lord Alli’s amendment, which would seek to maintain Britain within the single market as a member of the EEA.

“We will not be supporting the EEA amendment later this week.”

They add: “Staying in the EEA whilst outside the European Union would be the opposite of what the public voted for. There would be a huge democratic deficit, as the UK became a ‘rule taker,’ not a ‘rule maker’.”

“Crucially, despite suggestions to the contrary, EEA membership would require accepting free movement of Labour. Concerns about free movement were an important factor which drove our constituents to vote Leave, and almost half of all Remain voters wanted changes to free movement. We do not believe these concerns should be ignored.”

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The letter was coordinated by Stoke-on-Trent Central MP Gareth Snell, Stoke-on-Trent North’s Ruth Smeeth, shadow justice minister Gloria De Piero and former minister Caroline Flint.

It comes as a direct response to claims by pro-Europe MPs that there would be a parliamentary majority in favour of the UK joining the EEA if Labour backed it. That suggestion infuriated Labour MPs from Leave areas, who have made clear they would never vote in favour of EEA membership because it is likely to mean the continuation of freedom of movement – something unacceptable to many of their constituents. Some MPs fear the issue is already costing Labour support among working-class voters.

One shadow cabinet source told The Independent: “People like Chris and Chuka have been saying ‘if only the Labour front bench got behind the EEA, we’d win the vote hands down’. That definitely got a reaction from people who were saying, ‘Well, I wouldn’t vote for the EEA’. There were a couple of PLP meetings where it was very clear that people felt just as strongly against the EEA as they did for the EEA.”

In their letter, the MPs write: “We would also be unlikely to support any future amendment which seeks to keep the UK in the EEA after we have left the European Union, and would not support any change in Labour policy to that effect.

“We believe the amendment tabled in your name to the Lords amendment, which is supported by the majority of the PLP, more appropriately reflects Labour policy.”

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While at least 70 Labour MPs are expected to rebel against the party line in order to vote in favour of the EEA amendment, others plan to defy instructions and vote against it, causing a major breakdown in discipline as Labour MPs split three ways on the same motion.

Sir Keir Starmer, the shadow Brexit secretary, has been holding regular meetings with Labour MPs from Leave-backing constituencies to listen to their concerns. At one such meeting last week, MPs are understood to have emphasised the need for the party to stand firm in opposing a deal that would allow continued freedom of movement.

Some MPs from Leave-voting areas believe their voice has not been fully heard in the public debate and has instead been overshadowed by more vocal pro-EU colleagues dominating the media. The letter is, in part, an attempt to change this.

Mr Snell told The Independent: “This is a private letter being discussed with colleagues in town and coalfield seats where voterse supported leaving the EU.

“But we are all clear that the EEA is not the model which would work for our communities and would be unacceptable to our colleagues. It is quite clear that the strength of feeling in the PLP is such that even if a dozen Tories were to vote to support the EEA, a majority of Labour MPs would not do the same.”

The split centres on a cross-party motion passed by the House of Lords that would keep Britain in the EEA – in effect, the single market. MPs will vote on the amendment to the EU (Withdrawal) Bill when the bill returns to the Commons on Wednesday.

Labour has refused to support the amendment, with Mr Corbyn having insisted remaining in the EEA would be “not appropriate”. In a bid to offset a backbench rebellion, the Labour leadership has instead tabled its own amendment that would see Britain retain “full access” to the single market but not join the EEA.

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The internal row exploded in public on Sunday, when Chris Leslie, a vocal pro-Europe MP, accused Ms Flint of behaviour like leading Tory Brexiteer Jacob Rees-Mogg.

He told the BBC’s Sunday Politics: “The vast majority of Labour members, supporters, voters, all the polling shows ... want to stay in the single market because they know in their workplaces, this is going to matter.

“Caroline is starting to sound a little bit like Jacob Rees-Mogg with the sort of notion that we should almost go for Brexit at all costs.”

Ms Flint hit back: “I think when you are losing the argument, you go to desperate accusations.