A row has erupted between rival groups campaigning for Britain's exit from the European Union.
Vote Leave and Leave.EU are vying to be designated as the official Leave campaign when David Cameron calls a referendum.
Merger attempts appear to have been scuppered by personality clashes and differences over tactics.
Labour MP Kate Hoey has quit Vote Leave but will continue to co-chair the Labour Leave campaign.
A Labour Leave spokesman told the BBC it has now ended its affiliation to Vote Leave, an umbrella group which includes business leaders, Tory MPs and UKIP MP Douglas Carswell among others.
But Labour Leave's founder John Mills, who remains Vote Leave deputy chairman, said in a tweet that it was still affiliated to the campaign and would continue to work with all leave campaigners.
The latest bout of infighting was sparked by an internal letter leaked to The Times, by Mr Mills, a Labour donor, warning about "damaging bickering" at the top of Vote Leave.
In his letter, Mr Mills says feuding in the group had prompted the Labour MP's departure.
Ms Hoey is now supporting a third group, Grassroots Out, founded by two Conservative MPs and backed by Leave.EU, and is due to appear at a rally in Manchester later alongside UKIP leader Nigel Farage.
In his letter Mr Mills also criticises Dominic Cummings, a former special adviser to Michael Gove, who was the campaign's director, until a shake-up this week saw him moved to a non-executive role, accusing him of generating "ill-feeling".
Responding to Mr Mills' letter, a Vote Leave spokesperson said: "Vote Leave is a genuinely cross-party campaign and we value our Labour Party MPs and supporters across the country."
In a separate development. Green Party London Assembly member Dame Jenny Jones tweeted "will vote to leave #EU but cant work with an organisation with so little judgement as to put Lawson at its head," a reference to the appointment of Tory peer and climate change sceptic Lord Lawson as Vote Leave chairman.
The Electoral Commission has the task of designating the official leave and remain campaigns, which will get access to £600,000 in public funds, TV broadcasts, free mailshots and a spending limit of £7m.
The two main Leave groups differ on tactics and both are claiming to command more cross-party support - a crucial factor in getting the designation.
'Bickering can't continue'
Arron Banks, the UKIP donor who is bankrolling Leave.EU, has written to MPs to hit back at claims by Vote Leave that it is too focused on immigration, saying it is also stressing the positive benefits of quitting the 28 nation bloc.
He also attacks Dominic Cummings and Matthew Elliott, who ran the successful campaign against changing Britain's voting system in 2011's AV referendum, who also left Vote Leave's board this week for a non-executive role.
"I wouldn't put them in charge of the local sweet shop," Mr Banks says of the pair.
A Vote Leave spokesman said: "We've seen the letter and we wish him (Banks) well."
One of the founders of the Grassroots Out, or GO, campaign told BBC News the leave side of the debate "can't continue with all this bickering, there needs to be a coming together at some stage".
Conservative MP Tom Pursglove said: "It can't carry on as we are. We need to focus on winning the referendum."
He refused to say whether or not his group would be trying to win the official designation, but added: "If we're in a position where we can bring people together at the top then that's something we will look at. We've shown we can do that at grassroots."
Speaking on his regular LBC radio phone-in, UKIP leader Nigel Farage said he had tried to get the two campaigns to merge, but claims Vote Leave, which he describes as a "Tory front", "refuse to work with anybody".
In a message to all UKIP members and supporters, Mr Farage said the party was now officially behind Grassroots Out.
"With the unanimous support of our MEPs we now commit to supporting Grassroots Out and want our branches and members to join with GO as a means of working with activists from other parties and none."
He added: "So let us organise and mobilise our people's army around the GO banner for this referendum."
The row has been seized on by the pro-EU group Britain Stronger in Europe, which claims the out campaign has "descended into an all-out farce".
But exit campaigners say that group faces splits of its own, as the Scottish National Party will be running its own campaign and will not share a platform with David Cameron or other Conservatives arguing for Britain to stay in the EU.