LONDON (Reuters) - Former Prime Minister Tony Blair could return to a frontline role in British politics to try to prevent Theresa May's Conservative Party from destroying the country with a so-called "hard Brexit", he said in an interview.

The only Labour prime minister to win three general elections, Blair was hugely popular during the start of his 10 years in power but his support for the U.S.-led invasion of Iraq severely tarnished his reputation.

In an interview with Esquire Magazine, Blair said it was a "tragedy" that Britons were left with a choice between a Conservative Party intent on a hard Brexit and a Labour Party that he described as "ultra-left" and stuck in the 1960s.

"I don't know if there's a role for me," he said. "There's a limit to what I want to say about my own position at this moment.

"All I can say is that this is where politics is at. Do I feel strongly about it? Yes, I do. Am I very motivated by that? Yes. Where do I go from here? What exactly do I do? That's an open question."

Britain's shock vote to leave the European Union in June propelled May to power and the former interior minister has indicated that she sees the vote as a demand for wide-ranging change in the way Britain is run.

May's approach has spooked investors who believe Britain is heading for a "hard Brexit", meaning it will give up trying to remain in the EU's single market in order to impose controls on immigration from the other 27 member states.

The vote, which has triggered the deepest political and financial turmoil in Britain since World War Two, has also rocked Blair's Labour Party, with leftist leader Jeremy Corbyn accused of failing to campaign hard enough to keep Britain in the bloc.

First elected last year on a wave of enthusiasm for a new type of politics, Corbyn was forced to compete again for his job. Although he was returned as leader with a higher mandate than before, he still lacks the backing of the centrist members of his party.

Blair said Corbyn offered a "mixture of fantasy and error". As a result, he said Britain was a "one-party state".

"The reason why the position of these guys is not one that will appeal to an electorate is not because they're too left, or because they're too principled. It's because they're too wrong," he said.

"Frankly, it's a tragedy for British politics if the choice before the country is a Conservative government going for a hard Brexit and an ultra-left Labour Party that believes in a set of policies that takes us back to the Sixties."

Blair's chances of forging a new major role might be limited after a seven-year inquiry delivered a scathing verdict in July on his handling and justification for the Iraq War, with many Britons believing he should face criminal action.

(Reporting by Kate Holton; Editing by Richard Balmforth)