Former South Shields MP David Miliband says Labour must choose between backing a Tory Brexit or giving voters a final say on withdrawal.
The former foreign secretary attacked Jeremy Corbyn’s present Brexit position as out-dated and designed for opposition not power.
Writing in The New European, Mr Miliband said: “Labour needs to prepare for a life-changing question next autumn: will it vote to give a license for a Conservative Brexit, or will it insist that the voters be given a final say on the Brexit deal?
“The danger of sleepwalking to Brexit is greatest for Labour and Labour voters.”
Mr Miliband said Labour needed to sell the case for remaining in the single market and customs union on the grounds that membership of the institutions would safeguard equality and prevent a race to the bottom in rights and standards.
The call comes after Mr Corbyn provoked criticism from some pro-Europe Labour MPs when he insisted Britain must leave the single market after Brexit.
The former foreign secretary, who now heads the New York-based International Rescue Committee, branded Mr Corbyn’s outlook a “residue” from when the prospect of a Labour government seemed distant.
He said: “It is said that Jeremy Corbyn has changed in a fundamental respect since this time last year.
“He now believes he can and will be Prime Minister.
“But if so then his current position on Brexit is a residue, designed for opposition not for government.
“The party’s position is to say that they wish we had not decided to leave, but that since we have we should negotiate to keep all of the benefits of membership after we depart.”
Mr Miliband points out that, as French President Macron has recently pointed out, this is not on offer.
He added: “We can decide to leave, and get worse terms for our economic and political relationships than at present, or change our minds to preserve the privileges.
“But what we cannot do is leave and hold onto the advantages of membership.”
Last month Mr Miliband said Britain has been left embarrassed on the world stage after the Brexit vote and there was international “bafflement” the decision.
He said Labour should press for the option of continued membership of the EU to remain on the table and criticised Theresa May’s approach to triggering Article 50, starting the two-year countdown to Brexit, without knowing the outcome.