Labour shadow pensions minister Alex Cunningham has resigned from the post after breaking with party lines in a recent vote.
Mr Cunningham resigned on 20 December after voting against the Labour whip on an amendment to the EU withdrawal bill, alongside 61 other Labour MPs.
In a statement the MP for Stockton North said he felt the door to a possible membership of the customs union needed to be kept open to protect businesses.
On the final day of voting, the Labour frontbench whipped their MPs to abstain from voting in favour of a new clause which would allow the UK to remain in the customs union.
Despite 62 Labour MPs defying the whip and conservative MPs Ken Clarke and Anna Soubry voting in favour, it was defeated 320-114.
Today (5 January), the Treasury select committee released a statement from its chair Nicky Morgan MP revealing she wrote to chancellor Philip Hammond on 13 December 2017 asking if he would explicitly rule out the UK participating in a customs union with the EU as part of its end-state relationship.
The chancellor has declined to do so in his response dated 20 December, the same day Mr Cunningham resigned.
Commenting on the correspondence, Ms Morgan said it was widely thought that being in a long-term customs union with the EU had been ruled out by the government.
"But the chancellor’s letter confirms that this is not the case. It is vital that the Cabinet reach agreement on these central questions about the UK’s future relationship with the EU, as a matter of urgency.”
In a statement to FTAdviser today (5 January) Mr Cunningham said in recent months he has spoken with representatives from many companies in his constituency and beyond and received lobbying from many of them through other means.
“They are as one about the importance of the government delivering the kind of access and regulatory regime they need to continue their businesses effectively in Europe.
“I voted as I did as I believed I was leaving the door open for any deal with the EU to possibly include the membership of the customs union – something I felt was in the best interests of industry and jobs in my Stockton North constituency.”
Stockton North voted in favour of Brexit at the referendum in 2016.
Mr Cunningham said he respected the decisions and was determined to work for the best possible outcome for them in terms of jobs and rights.
Mr Cunningham had already stepped down from Jeremy Corbyn's shadow cabinet as minister for the natural environment in June 2016.
After being appointed as shadow pensions secretary in October 2016, Mr Cunningham used his first appearance to ask the government about the grievances of the Women Against State Pension Inequality (Waspi) campaign.
He called out the “shabby” treatment by the government of women born in the 1950s and demanded compensation for “some of the most needy women in our society”.