At Labour's annual conference last month, Burnham said Britain's departure from the bloc had done more harm than good.

In subsequent interviews he said it's possible a future Labour manifesto could include a pledge to hold an “in-out” referendum on rejoining the EU.

The U.K. prime minister addressed MPs after telling the Rest is Politics podcast in an interview aired over the weekend that it is “clear” Russia interfered in the 2016 Brexit referendum.

“I asked the question coming to Downing Street: ‘Was there interference in the 2016 referendum from Russia?’ And I’ve had the clear answer that yes, there was,” Burnham said.

But his spokesperson told reporters Monday:  "We do believe the referendum result to be legitimate."

Reform UK Leader Nigel Farage, one of the most high profile campaigners for Brexit in 2016, said Burnham's Russian interference claim was "outrageous" and accused the prime minister of being a "liar."