Voters who want to stay in the European Union need to back Labour, the party's shadow Brexit secretary has said on a campaign visit to Wales.
Keir Starmer said a new referendum can only come about if his party can form a UK government.
It comes after the Greens, Plaid Cymru and Lib Dems formed a pro-EU pact.
Mr Starmer said Labour's Brexit policy - to negotiate a deal and then offer it up at a referendum with Remain as the other option - is "very clear".
"Anybody who wants a referendum needs to vote Labour. Anybody who wants to fight for remain needs to vote Labour," he told BBC Wales.
"These questions are only live if we have enough people voting Labour to form an administration."
Mr Starmer was speaking on a campaign visit to Barry on Friday.
"An incoming Labour government would secure the best deal possible very rapidly," he said.
"We know what that looks like - it's a customs union/single market deal with protection for [workers'] rights and standards. That would be put up against Remain in a referendum within six months.
"You take the decision away from politicians, you give it to the public.
"People may agree or disagree with the policy but they can't argue that that's not clear," he added.
Welsh view 'respected'
Welsh Labour's position has been more pro-EU than the Labour leadership in London, saying if a referendum is held it would campaign to remain.
UK Labour plans to hold a conference after it secured a Brexit deal to decide how the party would campaign.
Mr Starmer said the Welsh Labour position is "respected".
"Many, many Labour party members and supporters will be in that position," he said, including himself among them.
"I would respect the position of somebody who wants to campaign the other way."