Sir John Major warns EU heading towards 'confrontation'
Former prime minister Sir John Major fears "confrontation" in the European Union as members in the eurozone "drift towards full fiscal union".
Writing in the Financial Times, he also predicts non-euro countries such as the UK, Poland and Sweden moving towards closer policy coordination.
David Cameron says he will review the balance of power between the UK and EU.
The prime minister also denied Labour claims the coalition is split on the issue of regaining powers from Europe.
His rebuttal came after opposition leader Ed Miliband said the prime minister "could not speak for the government" as he disagreed with his deputy, Nick Clegg, on how to proceed.
Mr Cameron joined other European leaders in Brussels for a crunch meeting on the eurozone debt crisis.
Meanwhile, in his FT article, Sir John - one of Mr Cameron's predecessors - says deeper eurozone integration "may encourage non-euro member states to seek to repatriate key policies they can't influence".
Sir John predicts a two-speed EU will emerge in which non-euro countries align themselves with each other, moving them away from nations which share a common currency.
He writes: "A more integrated eurozone will also provoke non-euro members of the EU by driving them further away from core decision-making... non-euro members will not wish to be marginalised and may sniff suspiciously at euro-core proposals, rendering decision-making even more of a hurdle.
"If the eurozone integrates and coordinates policy, non-euro members may co-ordinate too."
He goes on to say "outside [the eurozone], a looser union could emerge" and a "pattern of variable alliances is likely", leading him to conclude that "one thing is certain - the EU will not remain the same".
The former prime minister also turns his attention to the push for a referendum on the UK's membership of the EU, which garnered the support of 81 Conservative MPs in a vote in the Commons on Monday.
Sir John says "many are pressing for their nation to leave the EU", which he calls "an extreme option that would throw up far more problems than it would solve".
"For the UK it would be a dangerous mistake but, even so, our relationship within the EU will shift. Cool heads and clear minds are needed: our future depends on it," he says.