Prime Minister David Cameron is accused of being “knee-deep in a conspiracy” to keep Britain in the EU after leaked letters reveal he discussed the ‘remain’ campaign with big business when he was still officially on the fence over Brexit.

A leaked letter from security firm Serco’s CEO Rupert Soames to the PM reveals how the boss said he would look at how to “mobilize corporates” during the ‘remain’ campaign. The letter was sent 11 days before Cameron struck a deal with EU leaders over Britain’s place in the bloc. At that time, the PM said he could still back the ‘leave’ campaign if the UK failed at negotiations.

Former London mayor and prominent ‘leave’ campaigner Boris Johnson described the revelation as the “biggest stitch-up since the Bayeux Tapestry.”

In a letter to the PM, Soames – who is the brother of Tory MP Nicholas Soames and grandson of wartime PM Winston Churchill – said he was following up on a meeting between the two. The Serco CEO said he would contact FTSE 500 companies – Britain’s 500 biggest firms – to urge them to mention the risks of Brexit in their annual reports, to be published before the June 23 referendum.

“I am working with Peter Chadlington and Stuart Rose [head of the Britain Stronger in Europe campaign] with a view to contacting FTSE 500 companies who have annual reports due for publication before June and persuading them that they should include Brexit in the list of key risks,” Soames wrote.

The letter was dated February 8, a time when Cameron was officially on the fence over the EU referendum as he was fighting in Brussels to secure a “better deal for Britain.”

“I am not arguing – and I will never argue – that Britain couldn’t survive outside the European Union … If we can’t secure these changes, I rule nothing out,” Cameron told Parliament on February 3.

Leading ‘leave’ campaigner Johnson branded Cameron’s negotiations “meaningless.” “FTSE 100 chiefs are seeing their pay packets soar while uncontrolled immigration is forcing down wages for British workers,” Johnson said.

“Now we learn that some fat cats have been secretly agreeing to campaign for ‘remain’ while angling for lavish government contracts. It makes us look like a banana republic. And it is also now beyond doubt that the so-called renegotiation was a fiction designed to bamboozle the public.”

Labour MP Gisela Stuart, chair of the Vote Leave campaign, said Cameron had “serious issues” to address.

The revelations come a day after Chancellor George Osborne mocked ‘leave’ campaigners as conspiracy theorists. “George Osborne accused the ‘leave’ campaign of inventing conspiracies. Now we see that David Cameron is knee-deep in one,” Stuart said.