Labour’s deputy leader Tom Watson is to urge his party to “strongly” make the case for the UK staying in the EU.
In a speech later, he will reiterate his push for another referendum to break the Brexit deadlock – saying Labour’s “hearts are Remain”.
Leader Jeremy Corbyn has so far resisted calls to fully back another public vote, saying the party would support it in certain circumstances.
That position was blamed for Labour’s performance at the European elections.
The party came third behind the Brexit Party and the Liberal Democrats, with its share of the vote falling to 14%, and several senior figures criticised a lack of clarity on Brexit.
Shadow foreign secretary Emily Thornberry said those results boosted the case for another referendum – an outcome that Mr Corbyn has said is “some way off”.
Labour MPs remain split, with those representing Leave areas warning against backing a public vote.
In a speech to the Centre for European Reform, Mr Watson – who has repeatedly put pressure on Mr Corbyn to back a further referendum – will say there is still time to reverse the Brexit vote.
“Pro-European is who we are and who we have always been. Our members are Remain. Our values are Remain. Our hearts are Remain,” he will say.
“The only way to break the political deadlock is to bring the public back into this decision and we must argue strongly to remain.
“Our future doesn’t need to be Brexit. We can change our future. We can put Britain back at the heart of Europe again.
“We can be proud of leading the fight for a fairer and stronger future, together. But we can only achieve this future if Labour fights for it and champions it. It’s time we do that.”
BBC
