LONDON (AP) — British Labour Party leader Jeremy Corbyn is facing increasing pressure to change the party's Brexit stance ahead of a key speech on Britain's plan to leave the European Union.

More than 80 senior Labour Party figures signed a statement published Sunday in The Observer calling for Britain to remain in the EU's single market and the customs union after Brexit. The letter states that Britain's economy would be so damaged by leaving the single market and customs union that Labour would be unable to carry out its reforms if it comes to power.

The letter states that Labour's position in Parliament means it is in a position to prevent Britain from going too far in severing its economic ties to the other 27 EU members.

It says: "For the sake of building a better Britain and safeguarding those our party was founded to protect, we must grab this chance before it is too late. We will never be forgiven if we fail to do so."

Corbyn has until now steadfastly resisted calls to seek ways to keep Britain in the EU's single market and customs union after Brexit, but he is expected to soften his stance on the customs union in a speech Monday.

Keir Starmer, who advises Labour on Brexit matters, told the BBC on Sunday that Corbyn will announce the customs union policy shift Monday. He said the customs union's benefits to Britain's economy are clear because it would greatly ease trade with Europe once Britain leaves the EU, which is expected at the end of March 2019.

"Obviously it's the only way of realistically to get tariff free access," he said "It's really important for our manufacturing base. And nobody can answer the question how you keep your commitment to no hard border in Northern Ireland without a customs union."

If Corbyn and Labour speak out clearly in favor of a customs union, it would give the party a different approach from the one favored by Conservative Prime Minister Theresa May, who has said repeatedly that Britain will leave both the customs union and the single market.

Still, her own Cabinet has been badly divided on how best to pursue Brexit, and she is planning a major speech Friday to provide further details after a series of Cabinet meetings.

The letter published Sunday urges Corbyn to seek to keep Britain in the single market as well — but there are no indications he is ready to make that shift.