Alex Cunningham resigned his post as shadow pensions minister after a clash with the party’s hard-Left leader over whether the UK should stay in the EU’s customs union.
He was understood to have stood down before Christmas but the reason for his exit was only revealed today.
Mr Cunningham, who had been in his frontbench post for 15 months, defied Labour whips to vote in favour of Britain remaining in the EU customs union after Brexit.
He was among 64 Labour MPs who ignored their leader’s instruction to abstain in a Commons vote on the issue last month.
They backed an amendment to the Government’s EU (Withdrawal) Bill proposing continued membership of the customs union.
In a statement, Mr Cunningham insisted his vote had been “in the best interests of industry and jobs” in his Stockton North constituency.
He said: “In recent months, I have spoken with representatives from many companies in my 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.”
“They also spoke of the threat to investment and the future of many of the jobs they provide in my constituency and across Teesside.
“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.
“After my constituents voted overwhelmingly to leave the EU I saw it as my responsibility to support them in that decision and ensure I worked for the best possible outcome for them in terms of jobs and rights.
“That I will continue to do.”