Nigel Farage has told No campaigners to "stop moaning, stop bitching" or risk losing the EU referendum.
The UKIP leader said the No side needed to "get off its backside", with the Yes side "in full flight".
He said UKIP would not be applying to lead the official No campaign, but would "mobilise a people's army" in favour of leaving the European Union.
"We are taking the lead on the ground," he said and would launch hundreds of public meetings from September.
This was in contrast to "wait and see" tactics he said were being favoured by Conservative and Labour Eurosceptics.
Mr Farage told BBC 5 live he knew UKIP would not lead the No campaign and that there were two groups competing to be the official No campaign in the referendum on whether the UK should remain a member of the European Union.
But he accused some Conservative Eurosceptics of being "consumed by rather petty jealousies".
Mr Farage said his message to them was: "Stop moaning, stop bitching and get off your backsides and help us win this referendum."
No campaigns - By Robin Brant
With friends like these... that's what Nigel Farage may be thinking today after a No campaign set up and part financed by the man who gave £1m to UKIP last year made it clear it doesn't want him to front the campaign. www.theknow.eu is yet to officially launch, and it is only one of several No campaigns in the mix but it is up and running and its chief executive is distancing the operation from UKIP and its leader. Liz Bilney told me it would be "absolutely perfect" if Nigel Farage and his party want to "fall into line" behind the campaign she's running, but she wants it to be non-political.
Her longtime boss Arron Banks, who opened his chequebook for UKIP last autumn, agrees with her, she told me. In explaining that they want to be more "far reaching" in their appeal they echo the concerns that prominent UKIP figures Patrick O'Flynn and Suzanne Evans voiced in the aftermath of the election.
In a speech in London outlining the themes the party believes can win the EU referendum for the No side, Mr Farage stopped short of ruling himself out as leader of the official campaign.
But he said: "What we will do is play our part in this campaign. At the moment there are two competing bodies: one very much a Westminster club, one very much an outside the Westminster club.
"There's merit in what both of those organisations are trying to do. We as a party... will make our minds up about which of those teams we will back, although privately, I hope... there will be a coming together."
UKIP would work with anybody in the Eurosceptic movement, he said, adding: "There'll be no negativity from us once they've got off their backsides and decided to join this fight."
Asked if he would step back from a campaign role if his presence jeopardised the outcome of the vote, Mr Farage said "Well, obviously."
But he added: "Whatever my shortcomings are, and they may be many, I think I'm probably better than no-one in going out to do this [ground campaign]."
Responding to a question on whether Boris Johnson should take the lead, Mr Farage said the Tory London mayor and MP had the advantage of being well-known but would need a "damascene conversion".
"I would say, 'come on Boris, see the light, come on in the water's lovely'," he added.
In an interview with BBC Radio 4's Today programme, Mr Farage said Mr Cameron must not continue to go "unchallenged" in his EU renegotiations.
He said the PM's strategy was solely focused on migrant benefits, which he said failed to address public concerns about the free movement of people, sovereignty and the cost of EU membership.
The leader of the No campaign, Mr Farage argued, needed to be someone who could best take the fight to Mr Cameron on TV and radio and "make the arguments that real folk care about it".
"We cannot pretend, as some in Westminster are doing, that we can ignore the question of open borders; that, I think, in the end will be the key to who wins this referendum," he added.
Asked about Labour leadership hopeful Jeremy Corbyn, Mr Farage said that "underneath his pretence that he wants us to stay in the EU I suspect there is a Eurosceptic bursting to get out, so if he does win the leadership it will be very interesting".
In an interview with the BBC while in Vietnam, Mr Cameron said he wanted to get on with the EU referendum - which has to be held by the end of 2017 - but said there was "no point" in holding it until negotiations were complete.
'Now the negotiation is under way, technical talks are happening now in Brussels, trying to fix the things like getting out of ever closer union which we never wanted to belong to, making sure Europe's competitive, frankly making sure Europe speeds up and signs trade deals with places like Vietnam more quickly, so all these things will happen and then we'll hold the referendum," he said.
When asked if there were any circumstances under which he would recommend people to vote 'no' in the referendum Mr Cameron said he ruled "nothing out" if he didn't get the changes he wanted.