Union boss Len McCluskey will go to a face-to-face Brexit meeting with Theresa May later this week.

The Corbyn-backing leader of Unite accepted an invitation to discuss the prime minister’s Brexit plans despite the Labour leader refusing one.

Downing Street said the discussion is part of an on-going “process of engagement” as Ms May attempts to find a way to win backing for her deal.

News that Ms May had called Mr McCluskey last week raised eyebrows in Westminster because it was the first time she had spoken to him, or any union leader, since arriving at Number 10.

Her spokesman said on Tuesday: “The PM is due to be seeing a number of union leaders later this week, one of whom is Len McCluskey.”

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He added: “This is going to be a process of engagement with people from across the House of Commons, but also with other interested sectors.

“She is obviously speaking with business leaders today and she’ll be talking with trade union leaders later in the week. That obviously follows on from phone conversations that she has already had with them.”

Issues expected to be covered include employment rights and environmental standards, the spokesman said.

Jeremy Corbyn has declined an invitation to speak with Ms May, saying he could not contemplate talks until she ruled out the possibility of ever taking the Britain out of the EU without a deal.

The prime minister is expected to head back to Brussels to try and negotiate further concessions from the EU, particularly around the ‘Irish backstop’.

But it comes amid attempts from MPs, including ex cabinet Dominic Grieve, to take control of the Brexit process if Ms May cannot get her deal through parliament in a second vote likely to take place next month.

Mr Grieve’s amendment which will be voted on next week contains a provision for MPs to take control of parliamentary business one day every fortnight in February and then a day a week in March until Brexit day.

Liam Fox hints at changes to Ireland backstop issue in Brexit deal

A further cross-party group of MPs, including Ms Cooper and Tory former minister Nick Boles, is seeking to give time for a bill to suspend the Article 50 withdrawal process if there is no new deal with Brussels by the end of February.