John Redwood, a long-standing critic of the EU, has warned Eurosceptics that it would be unusual to form a "Brexit" campaign before the government has even secured its first substantial move towards holding a referendum on the UK's membership of the EU.

The senior Tory MP told IBTimes UK that he would concentrate his efforts on lobbying the government as a member of the newly formed Conservatives for Britain group.

Redwood stressed that the 50-MP-strong organisation is not part of the so-called Brexit campaign, despite the strong Eurosceptic leanings of some of its members.

"We are not an 'Out' campaign, the 'Out' campaign will form in due course. It would be unusual to form an 'Out' campaign before we even have an EU referendum bill put through the House of Commons," Redwood said.

The comments come as David Cameron continues to push for some key concessions from Brussels, including blocking EU migrants from receiving in-work-benefits when in the UK.

But Redwood said he had "no idea" if the prime minister could secure a better deal for Britain from the political and economic union. He added: "It depends on how much they want to keep us in, doesn't it?"

Cameron today (8 June) upped the stakes around the EU referendum and issued an ultimatum to Eurosceptic members of his all-Conservative cabinet, telling them they will have to resign if they campaign to leave the EU.

The prime minister, speaking at the G7 summit in Germany, said: "If you want to be part of the government you have to take the view that we are engaged in an exercise of renegotiation to have a referendum and that will lead to a successful outcome.

"Everyone in government has signed up to the programme set out in the Conservative manifesto."

The Tory leader stressed that his government "isn't neutral" on the issue and he will be hoping to secure a better deal for Britain.

Nigel Farage took to Twitter to react to Cameron's warning, claiming that the prime minister "wants to keep Britain inside the EU under any circumstances".

The Ukip leader has insisted that his party will "take the lead" of the Brexit campaign, with a series of events starting later this year.

"We are going to take the lead, we are going to get cracking. But we will at all times invite others to come along and share the platform with us," Farage told The Daily Telegraph.

"We will be launching a massive series of public events and meetings all over the country starting in September. These will be public meetings. They will be live web streamed.

"We are going to be busy, delivering leaflets through the doors by the million. We are not prepared to stand around and wait."