Ukip is starting the "ground war" for the campaign to leave the European Union, Nigel Farage has said.
The eurosceptic party leader said he wanted his party to be part of a wider coalition of Labour and Conservative politicians and leading business figures.
Holding the planned in/out referendum next year instead of 2017 would leave the No campaign struggling to get its message across without action now - and it was time to "get cracking", Mr Farage warned.
He told BBC Radio 4's Today programme: "If that happens, the No campaign simply wouldn't have time to organise and to mobilise."
Speaking ahead of Ukip's south east regional conference, Mr Farage said there was no point waiting for David Cameron to complete his negotiations on Britain's links to Brussels before starting to campaign.
He added: "The one thing Mr Cameron is not asking for, because he knows he won't get it, is an end to the total free movement of people.
"So, as far as we are concerned there is nothing Mr Cameron is asking for that could be acceptable."
Ukip will stage a "massive" series of public meetings as part of the campaign.
"We are the people that can fight the ground game. We're the people that can put leaflets through doors, we're the people who can put posters in farmers' fields.
"We're the people that can organise and hold a massive series of public meetings, which I'm going to be doing with my colleagues from September."
And Mr Farage added: "I'm saying we will take the lead in the sense we are going to get cracking. I am not saying Ukip must dominate the No campaign.
"I think we do need a group of businessmen, a group of cross-party people that form a No umbrella under which Labour, Conservative eurosceptics, sports stars, whatever it is, can operate.
"In the absence at the moment of that umbrella, what I'm saying is we are going to start the ground game.
"I'm not saying Ukip on their own can win this referendum, but what I am saying is that Ukip is a very important part and component of it."
Mr Farage called on eurosceptic Labour and Conservative MPs to stand up and say the Prime Minister "is simply not asking for enough" in the renegotiation.
He added: "My worry is that many of these well-known Tory eurosceptics, I suspect their loyalty to the Conservative party may in the end be greater than their loyalty to the country and to this cause.
"I can see a stitch-up around this term 'associate membership'. That's what Mr Cameron is going to come back with and, frankly, it won't mean much."
Eurosceptic Conservative MP Bernard Jenkin said he would wait to see how the renegotiations panned out but suggested he would campaign to exit if they failed.
He told the programme: "Unless we control things like migration, like how much we pay to the European Union from our own Westminster parliament, we are not actually making much of a change to our relationship.
"At that point I think Mr Farage and I might find that we have got more in common than he is prepared to admit at the moment."