British Prime Minister Theresa May will be given an indication of the strength of opposition to her Brexit plans as MPs in the House of Commons debate the decision to leave the EU customs union.
The debate comes after the Mrs May suffered a setback on the issue with an overwhelming defeat in the House of Lords and with a looming crunch vote on a customs union in the lower house.
The debate is non-binding and Downing Street has played down its significance, saying it was a "routine backbench business debate".
However, a binding vote on legislation due in the commons will be harder to dismiss.
Shadow Brexit secretary Keir Starmer has accused Mrs May of delaying the showdown over the Taxation (Cross-border Trade) Bill because she fears a humiliating defeat.
House of Commons library analysis commissioned by Labour indicated the decision to rule out a new customs union with the European Union could cost the UK economy £24bn by 2033.
Mr Starmer said: "Over the past few weeks it has become abundantly clear that Theresa May is unwilling and unable to put the country's interests first during the Brexit negotiations.
"She has wasted 12 weeks of the Brexit negotiations delaying a Commons vote on the UK negotiating a customs union with the EU for fear of a defeat.
"In light of the Government's own impact assessments and the lack of progress on any new trade deals, any economic case for ruling out a customs union has collapsed.
"The Prime Minister is now solely focused on internal party management and masking the divisions within her government."
Brexit minister Suella Braverman said Labour's support for a customs union "would leave the UK shackled to Brussels", taking rules set by the EU and unable to strike trade deals.
Labour leader Jeremy Corbyn has backed a customs union but only if the UK had a say in future EU trade deals.
Mrs Braverman said: "The supposed safeguards that Corbyn talks about are impossible to achieve and will get laughed out of town."
She said Labour was "more interested in frustrating the process and playing politics than they are in delivering a successful Brexit".
The motion being debated calls on the Government to include as an objective in negotiations with the EU "the establishment of an effective customs union between the two territories".
Signatories to the non-binding motion tabled by parliamentary committee chairs include four senior Tories: Bob Neill, Sarah Wollaston, Nicky Morgan and Dominic Grieve.
Labour Home Affairs Committee chairwoman Yvette Cooper and Treasury select committee chairwoman Ms Morgan said in a joint statement: "We both believe the case for a customs union is overwhelming - for the sake of British manufacturing, international trade, smooth borders and Northern Ireland peace."
