Former British prime minister Tony Blair has urged Labour supporters who can no longer vote for the party to endorse another which backs Remain.
He said Labour would be "hoovering up" Remain votes had it taken a "stronger position" on the issue, and he condemned a potential soft Brexit as unworkable.
Speaking to Sky's Sophy Ridge On Sunday, the former UK Labour leader said it is "important the anti-Brexit side is larger and stronger than the Farage side" in the European election later this month.
He said: "I do come across people who cannot vote for Labour, in which case I say 'don't stay at home - vote for any of the other parties'.
"At the end of this there's going to be a totting up and... it's important that the Remain side comes out because MPs will be making decisions based on that."
Mr Blair voiced frustration with Labour on Brexit, saying it is "clear they are not Remain in an unequivocal sense", although he said there is "enough" to make him continue to vote for the party.
He said leader Jeremy Corbyn should have been clear from the start that although Labour respected the referendum result, they would "reserve the right to put it back to the people".
He said: "Labour would be in a hugely stronger position today because it would be hoovering up all those votes that are presently for the Lib Dems, Greens, Change UK and the SNP, which adds up to pretty much 30%."
Mr Blair also attacked the soft Brexit position expected to come out of cross-party talks between the Government and Labour.
He said: "It's a fallacy because the soft Brexit Theresa May has tried to articulate and the Labour position won't work.
"That soft Brexit is never going to command anything other than a tiny amount of support.
"The people who want Brexit will say it's a betrayal and the people who want to remain will say it's completely pointless."
Mr Blair also warned that supporters of a no-deal Brexit are "extreme" and would "cause enormous damage to the country".
He said: "Nigel Farage and the people associated with him are not the people to drain the swamp - they are the people who created the swamp.
"They have created this as if it's the answer to our problems, and it is literally the problem."