Prime minister says he will block efforts by eurozone countries to make euro more effective unless they agree to his demands

David Cameron has claimed he is "entitled" and "enabled" to block efforts by eurozone countries to pass treaty changes that would create an effective currency union unless his EU partners agree to his own demands for treaty changes that would create a looser UK-EU relationship.

Cameron made his threat in an interview on the BBC's Andrew Marr programme on Sunday as speculation mounts on the contents of his landmark Europe speech later this month setting out his plans to demand a looser relationship with the EU.

He again said he did not favour the UK leaving the EU, arguing that countries outside the EU such as Norway are subject to the EU's rules but have no influence over its policies.

Setting out how Britain would have a lever over the rest of the EU to demand repatriation of UK competences, Cameron said: "What's happening in Europe right now is massive change being driven by the existence of the euro. The countries of the euro, they have got to change to make their currency work. They need to integrate more … This is something they recognise they have to do – there isn't a single currency in the world that doesn't have a banking union and forms of a fiscal union."

He added: "As they need to change, what that means is they are changing the nature of the organisation to which we belong. And thus we are perfectly entitled – and not just entitled but actually enabled, because they need changes – to ask for some changes ourselves."

Britain alone can veto treaty changes to make the euro more effective, such as closer EU supervision of banks and deficits. All these treaty changes require the unanimous support of all EU members, and euro members are likely to take a dim view of Britain threatening to block an effective euro as part of a bargain designed to allow Britain to opt out of other EU laws, especially laws designed to create social equality.

EU sources have also claimed there is no certainty that treaty changes will be required, and if they are, and Britain seeks to veto them, then the changes could be agreed inter-governmentally without the need to use EU institutions.

Cameron said he supported treaty changes in principle to make the euro more effective.

He said: "I'm very positive about the changes that they need to make, but I think it's a perfectly acceptable argument to say that as you need to make your changes, there are changes that Britain would like to make too. We want to be members of the European Union, particularly the single market, but there are changes we would like to make."

He insisted the rest of the EU would concede to Britain's as-yet-unspecified demands, saying: "I'm not a fatalist. People told me it was never possible to make changes to our relationship. I came in as prime minister, I've already managed to get us out of the bailout power that the last government opted us into."

Pressed to explain the kind of competences that Britain would seek to have repatriated, he said: "There are lots of things we would be better off out of."

He added: "The working time directive in my view should never have been introduced in the first place because it's actually affecting things like the way we run our hospitals rather than simply about business and trade and the single market."

He also repeated a warning by the home secretary, Theresa May, that he would like to make it more difficult for EU citizens to claim benefits in the UK if they are working in the UK, a change that might not need treaty reform.

He said: "One of the key reasons of being a member of the European Union are what are called the key freedoms: the movement of services, the movement of goods and the movement of people. Now, there are restrictions already on the movement of people if you have, for instance, an emergency. Should we look at arguments about, should it be harder for people to come and live in Britain and claim benefits? Yes, frankly we should."

He defended the Foreign-Office-led review of competences between the EU and the UK due to be completed by 2015. "We've got a process under way, a proper process with the balance of competences review where the government – and the public can be as involved in this as they like – are going through competence after competence, area after area, and saying: 'What is the balanced argument? What is right at the European level? What's right at the national level?'"

Cameron refused to be drawn on the exact nature of a referendum he would offer since his long-awaited speech on the issue is not far away, but reiterated there would be a "real choice" for Britons.

He argued against Britain leaving the EU, saying: "50% of our trade is with the European Union. At the moment, because we're in this single market, we have a seat at the table in the single market, we help write those rules. If we were outside the EU altogether, we would still be trading with these European countries but we would have no say."