Labour was accused of wanting to drag Britain back to the EU after Sadiq Khan made a fresh demand for closer ties.
The London Mayor called for a youth mobility agreement with Brussels and said the UK should consider rejoining the customs union.
But his comments have prompted new warnings from the Tories that Sir Keir Starmer would take the UK back towards the bloc.
Conservative Party chairman Richard Holden MP said: “It’s hardly surprising Sadiq Khan wants to drag us back into the EU given Sir Keir Starmer spent years trying to block Brexit then overturn the largest democratic vote in British history.
“And now he’s plotting to rip up our new relationship with the EU and even drag us back into the EU by ‘stealth’ with plans to hand millions of EU nationals the vote.
“Rishi Sunak and the Conservatives will continue to build on our freedoms and stick to the plan which is working and has seen us have higher economic growth than either France or Germany.
“It couldn’t be more clear that Labour would take us back to square one.”
Sir Keir backed Remain and a second referendum but now insists a Labour government would not look to rejoin the single market or customs union.
Speaking at the Fabian Society conference in London on Saturday, Mr Khan said: “I’m not saying today we should rejoin the EU.
“What I’m saying is that (shadow foreign secretary) David Lammy is on to something when he’s saying we should have a closer relationship with the EU.”
He added that re-entering the customs union should be on the table when the Brexit deal with the EU comes up for review in 2024.
Mr Khan also said a youth mobility agreement would benefit the UK “economically, culturally and socially”.
