It is a “shame” that pro-EU parties have not been able to agree on a joint platform for the European elections, Liberal Democrat deputy leader Jo Swinson said.
The May 23 contest will feature Lib Dem candidates battling against would-be MEPs from Change UK and the Greens who are all explicitly calling for a second referendum.
Outgoing Lib Dem leader Sir Vince Cable said the “fragmented” approach would result in fewer seats for the parties.
Ms Swinson said: “I think it is a shame that those Remain-supporting parties aren’t able, weren’t able, to come to an agreement to be fighting that together.
“But the wider aim of the securing of a people’s vote to stop Brexit is one where we are very much still working together in Parliament and outside of Parliament.”
Change UK confirmed it was working with the Lib Dems in Parliament to press for a referendum.
On BBC One’s Andrew Marr Show, Ms Swinson said the Lib Dems were “the original and best” because they had been calling for another referendum since 2016.
Change UK’s Chris Leslie has highlighted the “baggage” that the Lib Dems carry from their time in coalition with the Tories and suggested that the centre ground needed a fresh party.
Ms Swinson said “those comments are unhelpful” but “I think that we can have a positive working relationship” with other parties.
With Lib Dem leader Sir Vince stepping down, Ms Swinson is viewed as the frontrunner to succeed him.
Although she declined to confirm she would stand in the contest, she said: “I’ve got a lot of ideas for what the Liberal Democrats need to do, and there’s going to be a discussion within the Liberal Democrats – in the not-too-distant future.”
Sir Vince, who indicated he would not stand again as an MP if the next general election does not happen until 2022, told BBC’s Pienaar’s Politics the Remain-leaning parties would suffer as a result of the failure to reach agreement.
“The fact that we have a fragmented offering obviously weakens the effectiveness of the Remain campaign in the European elections,” he said.
On his own future, Sir Vince said: “It depends when the general election is. If there is an early general election, I have made it very clear I would stand again. We won’t have a new leader until July so I would be leading the party in any event.
“If we are talking about a general election on the normal timetable, no, I will be standing aside.”
Change UK’s spokesman Chuka Umunna set out plans for his party to have a series of rallies in the run-up to the May 23 election.
The party acknowledged calls for a “Remain alliance” at the European elections, but said that electoral law prevented such a deal unless parties merged and collaboration short of a formal union risked falling foul of the rules.
Mr Umunna said: “I can totally understand the frustration people have with the electoral system used to choose MEPs, which forces parties to merge if they wish to stand on a joint slate – that is why we need to change it to a better form of proportional representation.
“It is important that, short of merging, we don’t commit the mistakes of Vote Leave and BeLeave in collaborating in a way that breaks the law.
“But we can assure people we are working very closely with the Liberal Democrats and others in Parliament to campaign for a People’s Vote and to Remain, and have put in an application for a joint Change UK-Liberal Democrat Opposition Day debate on a People’s Vote.”
Change UK will be holding eight People’s Vote and Remain rallies in London, Norwich, Nottingham, Sheffield, Cardiff, Bath, Edinburgh and Manchester.