UKIP leader Nigel Farage has given his backing to both groups campaigning for Britain to vote to leave the EU.
Two separate campaigns, Vote Leave and Leave.EU, are pushing for an EU exit ahead of the referendum, before 2018.
Playing down suggestions of a split in the camp, Mr Farage said they were "complementary" not "contradictory" and predicted they would ultimately merge.
The In Campaign, which will make the case for continued EU membership, officially launches on Monday.
Mr Farage has previously said UKIP will stand "hand in hand" with the Leave.EU group - launched at UKIP's conference and funded by party donor Arron Banks.
But speaking to the BBC's Sunday Politics programme, the UKIP leader said both groups had his support, as they targeted "different audiences".
Vote Leave comprises a cross-party group of MPs and peers from the Conservatives and Labour, and UKIP's only MP Douglas Carswell. It is being run by Taxpayers' Alliance campaign group founder Matthew Elliot, who organised the successful 'No2AV' campaign in the referendum on Westminster's voting system, and Dominic Cummings, a former special adviser to Conservative cabinet minister Michael Gove. It has the backing of three existing Eurosceptic groups: Conservatives for Britain; Labour Leave and Business for Britain, and is being funded by party donors.
Leave.EU was formerly called The Know and rebranded when an amended referendum question was proposed. Founded by UKIP donor Arron Banks, it has been described by party leader Nigel Farage as an "umbrella group" of anti-EU campaigners. It describes itself as "Britain's fastest-growing grassroots organisation" and claims to have gained 175,000 members since The Know was launched in August.
Mr Farage said Vote Leave - which launched on Friday - was a "Westminster-based group" putting forward business arguments, while Leave.EU was "reaching out to millions of ordinary people" and making the case for Britain being able to control its borders.
UKIP's only MP, Douglas Carswell is supporting the cross-party Vote Leave campaign.
But Mr Farage said: "My view is that I will support both. I'm not interested in being partisan about this and I'm really confident that at some point in time they will all come together."
Meanwhile, Labour MP Kate Hoey told BBC Radio 4's The World This Weekend programme there was increasing support among trade unions for Britain to leave the EU.
There was a "growing recognition and acceptance" that the EU is not in the interests of workers' rights, she said. "It's not in the interests of ensuring that we can trade with the rest of the world", she added.
In depth: The UK's EU referendum
Prime Minister David Cameron has said he will campaign for Britain to remain in the EU - if he secures the reforms he wants.
Former immigration minister Damian Green told The World This Weekend that Mr Cameron would not to return "empty-handed" from his negotiations in Brussels.
"I don't think there's a chance that he'll come home empty-handed, not least because some of the reforms he's trying to negotiate aren't just in Britain's interest, they're in Europe's interest as a whole," he said.
'Sooner the better'
Monday will see the official launch of the cross-party campaign for continued EU membership, which is to be led by peer and former Marks and Spencer boss Stuart Rose.
Mr Farage said it would be "a group of yesterday's men and big corporate business".
But Innocent smoothies founder Richard Reed, of the In Campaign, told the Andrew Marr programme it was "a people's campaign" with a diverse board that included trade unionists, businesses, entrepreneurs, environmentalists, students and politicians.
Appearing on the same programme, businessman Richard Tice, from rival group Leave.EU, said the campaign was deliberately "non political" and "very much for the people". Leave.EU had signed up 200,000 since its creation, he added.
Meanwhile, speaking to the BBC's Andrew Marr show, former Conservative cabinet minister and pro-European Ken Clarke said "the sooner we have that referendum the better".
Increased spending limits
A date has not yet been set for the in/out vote but the government has promised it will be held by the end of 2017.
The elections watchdog, the Electoral Commission, has yet to designate the official campaigns on either side of the EU debate.
The groups designated as the official campaigns for the referendum will benefit from increased spending limits of £7m during the campaign period, campaign broadcasts and a free mailout to households.
The Electoral Commission says it will choose the campaign which represents it will choose the campaign which represents "to the greatest extent those campaigning for that outcome".