Tony Blair: Labour and Tories 'should be worried' about the Liberal Democrats

Former Labour Prime Minister Tony Blair. Picture: PA

He told BBC Radio 4's Today programme: "What will destroy the two-party system is if it becomes clear that the two main parties have moved so far away from the centre that the gap in the centre has to be filled in order to be representative of the state of opinion."

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He added: "You only have to look at the Liberal Democrats now, at their party conference. For the first time in a long time, they are looking a much more serious group of people."

Mr Blair said: "They have got a coherent argument. If I was the two main parties at the moment, I would worry a lot about that."

He was being interviewed on what was John Humphrys last day as presenter of the programme.

Humphrys will now bow out of the early morning show after 32 years.

"I should have gone years ago, obviously I should have gone years ago," he said, "but I love doing the programme."

Also interviewed this morning was former Conservative Prime Minister David Cameron.

Mr Cameron defended calling the EU referendum, saying it had come from "honest" motives.

He told the programme there had been "growing problems" with the EU and said there was a growing appetite for a referendum in the UK.

The former PM acknowledged he had "failed" with improving the situation but denied being "complacent", saying he takes a "big share of responsibility" for what has happened since.

Mr Cameron said he had fought hard during the referendum campaign, but had done so alone - claiming the Labour leadership had not been committed.

He also expressed his sadness that prominent former Tories like Ken Clarke and Nicholas Soames have been ejected from the party.

Speaking on the Today programme, Mr Cameron said Boris Johnson's focus must now be "100 per cent" on getting a deal from Brussels.