Vote Leave are trying to "exclude" Nigel Farage from their EU referendum campaign, Ukip claims.
The Ukip leader is set to go head-to-head with Prime Minister David Cameron in the first in-out debate live on ITV on June 7 - but this is being threatened with legal action by Vote Leave.
A Ukip spokesperson described this as an attempt at "excluding" Mr Farage from their campaign.
"It is deeply disappointing that rather than rallying behind Nigel Farage for what will be the biggest one-on-one debate of the referendum campaign, Vote Leave are instead threatening court action to stop Nigel from taking on the Prime Minister", the spokesperson said.
The statement added: "This referendum is bigger than the Conservative Party and bigger than party politics.
"It is about the very future of our country and this ITV debate will reflect that.
"All on the Leave side must put their egos to one side and support Nigel as he prepares to take on the Prime Minister in what will undoubtedly be the defining moment of the referendum campaign".
A source from the Vote Leave campaign said: "This is an outrage.
"ITV admitted to us that the only reason they were asking for Nigel Farage was to secure the Prime Minister.
"They are allowing Number 10 to choose their opposition. This is because Number 10 are refusing to debate leading Vote Leave figures.
"ITV have effectively become part of the 'In' campaign. We will take them to court and we will win."
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