Tony Blair has warned David Cameron's stance on Europe risks economic chaos and threatens the UK's national interest.

The former Labour PM also told Sky News current party leader Ed Miliband had his "100%" support, amid rumours of a rift between the two men.

Mr Blair said: "I believe passionately that leaving Europe would leave Britain diminished in the world, do significant damage to our economy and, less obviously but just as important to our future, would go against the very qualities and ambitions that mark us out still as a great global nation."

He also said an in/out vote would be a "huge distraction" and an "unacceptable gamble" over Britain's future.

Speaking in his former constituency of Sedgefield, Mr Blair added: "And the oddest thing of all about David Cameron's position? The PM doesn't really believe we should leave Europe; not even the Europe as it is today.

"This was a concession to party, a manoeuvre to access some of the UKIP vote, a sop to the rampant anti-Europe feeling of parts of the media. This issue, touching as it does the country's future, is too important to be traded like this."

He went on: "It is greatly to Ed Miliband's credit that he resolutely refused to make that trade. He faced down calls to follow the Tory concession from the media and many inside our party. In doing so, he showed real leadership."

The former Prime Minister will today call on the country to support Ed Miliband, in a sign the leading lights of New Labour are prepared to throw their weight behind his campaign.

Mr Blair is a controversial figure within the Labour party, despite winning three elections. In 2011 delegates at the Labour conference booed when Mr Miliband mentioned his name in a speech.

He was an ally of David Miliband, who lost to his brother in the 2010 leadership battle.

Mr Blair will deliver his speech alongside his wife Cherie Blair and Phil Wilson, his successor as Labour MP for Sedgefield.