London, United Kingdom: Britain's former prime minister Tony Blair said Friday there was nothing preventing a second Brexit referendum and warned that the break-up would be "very, very tough" as European leaders were in an unforgiving mood.

He called on fellow "Remain" supporters to "mobilise and to organise" against proponents of Brexit, writing in The New European newspaper that "we're the insurgents now".

"There is absolutely no reason why we should close off any options," he later told BBC radio.

"We are entitled to carry on scrutinising, and, yes, if necessary, to change our minds. This is not about an elite overruling the people.

"If it becomes clear that this is either a deal that doesn't make it worth our while leaving; or, alternatively, a deal that is going to be so serious in its implications that people decide they don't want to go, there's got to be some way, either through parliament, or through an election; possibly through another referendum," he added.

The Downing Street office of Prime Minister Theresa May immediately rejected the suggestion, with a spokesman stressing "there will be no second referendum, Britain is leaving the European Union."

"Tony Blair is entitled to put his views to whom he so chooses," said the spokesman, according to the Press Association.

"But what's important is the PM has been absolutely clear -- the British people have spoken, we are listening, we're going to leave the European Union."

Blair, who was prime minister from 1997 to 2007, called Brexit as "catastrophe", and revealed that he had recently held talks with French President Francois Hollande that had highlighted the challenges Britain faces in upcoming negotiations to set the terms of the break-up.

"It convinced me that it's going to be very, very tough," he said in The New European. 

"We are not going to be conducting this negotiation with a group of European businessmen who may well decide that what they want is the maximum access into the UK, and they may be prepared to be quite forgiving.

"The people we will be conducting this negotiation with will be the political leaders of the European Union, and their parliaments, so this is going to be a negotiation, in my view, of enormous complexity."

He said that Britain would either have to accept obligations in return for access to the single market of face "severe" economic consequences.

Conservative MP Maria Caulfield, a member of parliament's Brexit scrutiny committee, said Blair was partly responsible for Brexit by encouraging "uncontrolled immigration" while prime minister and that he was now unable to "come to terms with the decision of the people of the UK".

"Instead of desperately trying to find ways to thwart the will of voters and talking down Britain's prospects, Labour should be concentrating on helping to make a success of Brexit," she added.