PM says he is confident of striking deal on EU reform that would be legally binding and do ‘what it says on the tin’

David Cameron has given his clearest warning that he will sack any government minister who wants to campaign to quit the European Union, insisting the government will not be neutral once he has struck a deal on a new relationship with the EU.

Cameron's threat to sack ministers who back EU exit - reaction: Politics live Read more

He said he was confident of striking such a deal, which would be legally binding and do “what it says on the tin”, a formulation that leaves open whether the deal will stop short of immediate treaty changes.

Cameron’s decision to make clear he will impose ministerial collective responsibility, in contrast to the more liberal stance taken by Harold Wilson during an in/out referendum in 1975, comes amid the first signs of organised opposition to EU membership on the Tory benches.

A group of 60 Tory MPs have signed up to Conservatives for Britain, warning that they will campaign for an exit unless the prime minister toughens his negotiating stance on issues such as national sovereignty.

At the G7 summit in Bavaria, Cameron was asked whether he had “absolutely closed his mind to allowing ministers a free vote”. He replied: “I’ve been very clear. 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.”

When asked whether anyone in government who opposes that position would have to resign, the prime minister said: “Everyone in government has signed up to the programme set out in the Conservative manifesto.”

He added: “If I can get a position where Britain would be better off in a reformed Europe then obviously that is not something the government is neutral about. It’s not a sort of ‘on the one hand, on the other hand’ approach. If I can secure what I want to secure, I will have secured what I think is the right outcome for Britain.

“I am carrying out a renegotiation in the national interest to get a result that I believe will be in the national interest. I’m confident I can get that.”

Cameron dismissed the Conservatives for Britain threat, saying: “It’s not any one political party or MP that will decide, it will be the British people. It’s an in/out referendum where everyone’s vote counts the same.”

Eurosceptic MPs are to express their anger when the Commons debates the bill paving the way for a referendum on Tuesday.

Cameron’s relatively hardline position opens the risk that some prominent government members, including some in the cabinet, will quit to voice their opposition to staying in the EU. But following his re-election, Cameron is at probably the strongest point of his premiership.

He clearly feels either that he will strike a deal that Eurosceptic government members can stomach or else that the European issue is potentially too divisive to give Conservative ministers any licence to air their differences.

Cameron also suggested, contrary to the view of the Electoral Commission, that he thought it was perfectly feasible to stage the referendum on the same day as other domestic elections, a position that makes May 2016 the most likely date for the vote.

He said he wanted to see the rules on government campaigning in the referendum relaxed, arguing that the proposed rules were so restrictive that he might be debarred during the campaign period from even making a prime ministerial statement to the Commons after a meeting of the European Council. The Electoral Commission has said it would be wrong to allow civil servants to publish information on behalf of the government during the campaign.

Cameron hinted he may put a deal to the vote that may need further subsequent changes. Asked whether he would put a deal to the British people only once it was totally battened down, Cameron said: “The deal’s got to be clear. It’s got to be legally binding. It’s got to do what it says on the tin. That’s what I’m shooting for.”

Such a two-stage deal would make it easier to win French and German support for the UK’s demands since they could be integrated later with a wider package to reform the oversight and conduct of the eurozone, a reform requiring treaty change that is likely to be delayed until after the French presidential elections in 2017.

The foreign secretary, Philip Hammond, speaking on the BBC, said it was important to secure treaty change to protect Britain from “judicial attack” from the European court of justice.

He said: “We think we do need treaty change, we think some of the changes – in particular [those] we are demanding around availability of welfare benefits for new migrants from the EU – can only be sustained from judicial attack in the European courts. It is not treaty change for its own sake, it is treaty change in order to protect the real material changes we need to get from judicial attack.”

Barack Obama has urged the UK to remain inside the EU, saying “one of the great values of having the UK in European Union is its leadership and strength on a whole host of global challenges”.