Caroline Flint confirmed she would defy her party whips and abstain on the European Union (Withdrawal) Bill at its second reading.
Labour is seeking to reject the Bill, with votes expected after midnight.
The Bill will transpose relevant EU law onto the UK statute book to ensure there are no gaps in legislation at the point of Brexit.
"The truth is, whoever was in Government, we would have to pass a Bill of this kind to prepare for leaving the EU in March 2019," Ms Flint said.
"And there can be little disagreement on that, unless your ambition is to thwart the result of the EU referendum and prevent or delay the UK leaving the EU.
"Now I believe Labour's job is to improve the Bill by amending it - not killing the Bill at the beginning of its passage through Parliament."
She added: "I will work with others to improve this Bill, but tonight I cannot vote to block this Bill and I shall be abstaining to allow the Bill to be further discussed and amended.
"We have a job to do to ensure a smooth, orderly Brexit."