Chancellor George Osborne has warned a new settlement with Brussels must ensure fairness for the "euro outs" as David Cameron continued his drive to build support for his re-negotiation plan.
Mr Osborne said a key aim of the Government's re-negotiation of Britain's EU membership terms was the protection of the interests of countries like the UK which were not part of the single currency.
In his annual Mansion House speech to the City, Mr Osborne said that the Government was also determined to ensure that the integrity of the single market was preserved.
"We need a settlement that recognises that while the single currency is not for all, the single market and the European Union as a whole must work for all," he said.
"So among the principles we seek to establish in this re-negotiation are these simple ones: fairness between the euro-ins and the euro-outs enshrined, and the integrity of the single market preserved.
"It's in our interests that the euro is a successful, strong currency. So we're prepared to support the eurozone as it undertakes the further integration it needs.
"But in return, we want a settlement between the UK and the eurozone that protects the single market and is stable, fair and lasts."
His comments came as Mr Cameron was seeking to explain his plans to EU leaders attending a summit of European, Latin American and Caribbean nations in Brussels
The Prime Minister is expected to have further talks today in the summit margins with leaders from Spain, Belgium, Finland and Romania.
British officials confirmed that Britain's re-negotiation was now on the draft agenda for the next full EU summit starting in the Belgian capital on June 25.
Arriving in Brussels yesterday, the Prime Minister - who has recently conducted a whistlestop tour of key European capitals including Berlin and Paris to explain his thinking - was upbeat about the prospects.
"That reform and re-negotiation, it's under way, I'm content with the progress we've made so far," he said.