A leading Labour figure has withdrawn support for the Vote Leave campaign.
Ulster-born Vauxhall MP Kate Hoey said she no longer endorsed the group’s bid to be officially designated as the ‘Out’ campaign for the EU referendum.
Green peer Baroness Jones also said she was standing back from the organisation because it lacked “judgment”.
The developments prompted Ukip leader Nigel Farage to warn that the national poll would not be won without “left-wing support”.
The moves emerged after a leaked email was published from businessman John Mills to Vote Leave campaign director Dominic Cummings.
According to the Guardian and the Times, a message from Labour supporter Mr Mills on Tuesday accused the combative former Tory adviser of “generating more and more ill feeling”.
A day later Mr Mills was demoted to deputy chairman of Vote Leave and ex-chancellor Lord Lawson brought in to take his place.
Long-term Eurosceptic Ms Hoey was said to have stepped down as co-chairwoman of Labour Leave - which is associated with Vote Leave - over the feuding.
She posted on Twitter: “Press stories about me having left Labour Leave are nonsense. I will not be supporting Vote Leave for designation by the Electoral Commission.”
Baroness Jones wrote: “Will vote to Leave #EU but can’t work with an organisation with so little judgement as to put Lawson at its head.”
Businessman Arron Banks, a Ukip donor and founder of the rival Leave.EU group, says his requests for a merger with Vote Leave have been rejected.
In a round-robin letter to Tory MPs, he urged them to support the cross-party Grassroots Out (GO) campaign founded by Ms Hoey and Conservative backbencher Peter Bone.
“It is clear that the Labour Leave people have been treated with absolute contempt. To promote Lord Lawson in John’s place only compounds the humiliation,” he wrote.
“Vote Leave now has no real cross-party support, and I would urge you to consider supporting GO.”
Mr Farage - who insists he is not exclusively aligned to either group - tweeted: “Baroness Jenny Jones has now left Vote Leave. We need left-wing support if Leave side is to win.”
He added: “Labour figures such as Kate incredibly important if Leave side is to win this referendum.”
A Vote Leave spokesman said: “We enjoy cross-party support and we are working with both Labour MPs and supporters.”
Asked about Mr Banks’s letter, the spokesman said: “We wish Arron well.”
Ukip leader Mr Farage expressed “grave doubts and reservations” about Vote Leave as he urged his supporters to back the Grassroots Out campaign.
In an email to Ukip members and supporters he said he had “tried my hardest” to broker a merger between Leave.EU and Vote Leave but it had been impossible.
“As for the two groups that are contesting the designation, our MEPs’ view was clear: we congratulate Arron Banks and Richard Tice for bringing a fresh approach to British Politics with their Leave.EU campaign.
“They now have approaching 500,000 supporters and they are bringing new people into this campaign who have never been interested in politics on a large scale.
“They’ve also shown a willingness to work with and help anyone that is on our side. We wish them well with their campaign.
“I have to tell you that myself and the Ukip MEP group have grave doubts and reservations about Vote Leave. They do not believe that open borders, immigration and our security are key to this referendum.
“All polling shows that they are wrong.”
Urging support for GO, he said: “This is the way forward.
“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: “Let us organise and mobilise our people’s army around the GO banner for this referendum.”