David Cameron has told Tory eurosceptics that he will call a referendum on Britain's relationship with the European Union if the country is threatened by further loss of sovereignty to Brussels.

The prime minister's move – a change of tone rather than actual policy – comes as former defence secretary Liam Fox prepares to tell Tory activists that a re-negotiation of the country's relations with the EU is necessary and "life outside the EU holds no terror".

Cameron is under mounting pressure from Tory backbenchers to hold a referendum as members of the eurozone move closer to a full fiscal and political union in response to the crisis engulfing the single currency.

The Tory right argues that the UK's relationship with the EU will be changed fundamentally as a new "core Europe" takes shape and that the community, and Britain's role in it, will be different from now on.

At a press conference after an EU summit on Friday, Cameron disappointed eurosceptics in his own party by playing down the idea of an "in/out" referendum and insisting he had protected the UK's interests at the meeting.

At the summit EU leaders agreed to use the permanent bailout fund to support ailing European banks. Cameron insisted that during the negotiations he had protected UK interests.

However, in an article for the Sunday Telegraph, Cameron goes out of his way to stress that, were new treaties to be negotiated down the line, affecting the shape of the future Europe, a referendum might well be necessary.

Cameron says Britain is becoming swamped by EU legislation and bureaucracy, which he would like to see scrapped, and says fundamental changes to the EU's workings that threaten its sovereignty would need "full-hearted support of the British people". He adds: "For me, the two words 'Europe' and 'referendum' can go together."

Some 100 Conservative MPs wrote a letter to Cameron last week demanding a referendum on Britain's relationship with the EU to be held in future years. Cameron argues that an in or out referendum is not the right choice now because the "vast majority of the British people" want changes to the current relationship with the EU, rather than to leave outright.

Cameron argues in the article that Europe is encroaching ever more into British life. "The fact is the British people are not happy with what they have – and frankly neither am I. Put simply, for those of us outside the eurozone, far from being too little Europe there is too much of it. Too much cost, too much bureaucracy, too much meddling in issues that belong to nation states or civil society or, indeed, individuals.

"Whole swaths of legislation covering social issues, working time and home affairs should, in my view, be scrapped." He says that "tactical and strategic patience" is needed, stressing he is in coalition with the pro-EU Liberal Democrats.

He adds: "How do we take the British people with us on this difficult and complicated journey? How do we avoid the wrong paths of either meekly accepting the status quo or giving up altogether and preparing to leave? It will undoubtedly be hard going, but taking the right path in politics often is.

"As we get closer to the end point we will need to consider how best to get the full-hearted support of the British people, whether it is in a general election or a referendum."

Fox's intervention will be seen as an appeal to the Tory right. He is expected to say that now is the time to confront the change in Britain's relations with Europe and heal a running sore once and for all. The Tories' current position is that they will grant a referendum if and when there is a substantial transfer of sovereignty from the UK to the EU, which Cameron insists is not yet in the offing.

Cameron's shift in tone will place further pressure on Ed Miliband to commit his party to an in/out referendum in Labour's election manifesto for the next election, due in 2015.

Several members of Miliband's shadow cabinet, including his new head of policy, Jon Cruddas, are known to support an in/out referendum, believing that Labour must make the commitment before the European elections in 2014. Cameron won plaudits from eurosceptics in December when he wielded Britain's veto at a summit to thwart plans for a new treaty on closer EU integration because of insufficient safeguards for the City of London.

While his move was welcomed by the Tory right it angered the Lib Dems.