MPs are set to vote on the Article 50 bill that will pave the way for leaving the EU.
Shadow Welsh secretary Jo Stevens has quit the front benches and intends to defy party leader Jeremy Corbyn's instruction and vote against it.
But Aberavon MP Stephen Kinnock said party members should "suspend our disbelief" and support the bill.
"I campaigned passionately for Remain and am a committed pro-European," he told BBC Radio Wales' Sunday Supplement programme.
"I feel the UK's interests are best served being as close as possible to our European partners."
The area he represents - as part of Neath Port Talbot - voted by 56.8% to 43.2% to leave in last June's referendum.
However, Mr Kinnock said his decision is not based solely on that result, but because the UK as a whole voted to leave.
Jo Stevens' constituents in the city of Cardiff voted by 60% to 40% to remain and when she quit the front benches, she said a great number had told her to oppose the move.
While Mr Kinnock said he "absolutely respects" her position, he added: "We must suspend our disbelief and trigger Article 50.
"We have taken a massive risk with the future of our country by voting to leave.
"But we can't go back over that ground and have to move forward."
He said politicians are simply second guessing each other and positions will only really become clear when negotiations start with the other EU states.
If these go well, and access to the single market is achieved,  MPs can potentially accept the deal when they vote on the final terms in 2019, he said.
But Mr Kinnock warned that negotiations would be "incredibly difficult".
"I hope Theresa May is ready for a bumpy ride, as that is what she will get," he added.
A Labour MP who described himself as "a committed pro-European" said he will not vote against triggering Brexit.