The businessman heading up one of two EU "out" campaigns says he believes it is "highly unlikely" they will merge.
Arron Banks, co-chair of Leave.EU, said he would like to merge with Vote Leave but claimed they did not want to.
The Bristol businessman said relations between the two were "strained" but his group was "ready" for the referendum.
Both groups are vying for the right to be designated the official "out" campaign, which will benefit from increased spending limits.
No date has been confirmed for the referendum on the UK's membership of the EU, promised by the end of 2017, although there has been speculation it could be this year.
'Silly' spy claims
The Electoral Commission has yet to select a designated lead campaign for either the "in" or "out" sides.
Mr Banks said his group had "offered to come together" with Vote Leave - a cross-party group made up of MPs and donors - but they were "not particularly interested".
He told BBC Sunday Politics West: "I think it's highly unlikely right now. We would like to but I don't think they [Vote Leave] are in the mode of wanting to merge.
"They're an SW1 [Westminster] campaign. We don't believe it can be won from SW1 we think it's got to be won on the streets of Britain."
But he dismissed newspaper reports that Leave.EU had hired security experts to spy on the rival group as "silly".
The official "in" and "out" campaigns will be allowed to spend up to £7m during the campaign period and will be entitled to TV broadcasts and a free mailout.
The Electoral Commission says it will choose the campaign that represents "to the greatest extent those campaigning for that outcome".
Vote Leave is supported by Business for Britain, the Labour Leave campaign and Conservatives for Britain.
It has yet to comment.