Former British prime minister Tony Blair said there was a 50-50 chance of getting another Brexit referendum as Prime Minister Theresa May will be unlikely to secure a majority for any divorce deal in parliament.

"Whatever Brexit is on offer today is going to result in significant economic harm," Mr Blair, who served as prime minister from 1997 to 2007, told Reuters.

"I still believe it is possible that Brexit is stopped, I think there is no majority in parliament for any proposition that the prime minister brings back," he said, adding that he wanted a second referendum.

Less than six months before the United Kingdom's exit from the European Union, there is little clarity about how the world's fifth largest economy and its preeminent international financial centre will trade with the EU after Brexit.

Mrs May is trying to clinch a deal but there is uncertainty on whether she could sell it at home, where she will need approval from the British parliament.

Mr Blair said European regulators would not want the centre of European finance to be outside their orbit so jobs would be lost in the financial sector.

"Why give ourselves this problem in a field of the global economy where we are globally preeminent?" Mr Blair said, adding that the government had cast aside the interests of the service sector.

Mr Blair said that if Brexit did go ahead and was followed by a Labour government led by Jeremy Corbyn, then the country would face a "truly damaging and challenging situation".

"This is the problem with the policies of both major parties: they seem to think you can do Brexit and then engage in a whole lot of social legislation to make capitalism fairer and more equal and so on," he said.

"They have got to wake up to the fact that if you do Brexit your number one priority is going to be keeping this place as an attractive place for investors to come."