The claims centre around Vote Leave – the officially designated campaign in the 2016 referendum – and its links to the BeLeave group that it helped fund. Whistleblower Shahmir Sanni, who worked on the campaign, claimed Vote Leave used BeLeave to get around strict spending limits set by the Electoral Commission. Vote Leave has strongly denied wrongdoing and said the £625,000 donated to BeLeave was within the rules.
But Mr Sanni said: “I know that Vote Leave cheated ... I know that people have been lied to and that the referendum wasn’t legitimate.”
Cabinet ministers Mr Johnson, Michael Gove and Chris Grayling were among senior politicians involved in the Vote Leave campaign.
Mr Johnson said stories based on Mr Sanni’s testimony were “utterly ludicrous” and Vote Leave “won fair and square – and legally”.
Mr Gove, who was campaign co-chair for Vote Leave, said the result of the referendum must be respected.
“I respect the motives and understand the feelings of those who voted to remain in the EU,” he said. “But 17.4 million opted to leave in a free and fair vote and the result must be respected. It’s our job now to work to overcome division.”
Deputy Labour leader Tom Watson told BBC1’s Andrew Marr Show: “Theresa May needs to make sure the Electoral Commission has the resources to fully investigate the allegations made that there was criminal collusion.
“Because let’s remember, the people that led these campaigns are now senior Cabinet members and I think we need to make sure that they were not aware of what was going on and that’s why I think the resources are needed and if needs be the police should be resourced to investigate as well.”
Liberal Democrat Brexit spokesman Tom Brake also suggested the police should be called in. The allegations centre on Mr Sanni’s claims BeLeave was controlled by Vote Leave rather than an independent campaign.
Mr Sanni said: “In effect they used BeLeave to over-spend and not just by a small amount ... almost two-thirds of a million pounds makes all the difference and it wasn’t legal.”
Mr Sanni was in a relationship with senior Vote Leave figure Stephen Parkinson – now Theresa May’s political secretary – at the time of the referendum and claimed through his lawyers he was “outed” by the Number 10 aide in the run-up to his election spending disclosure. Mr Parkinson said he was “saddened” by the “factually incorrect” statements by Mr Sanni.