Senior Brexiteers working to stop Theresa May's Chequers deal have started to take precautions amid concerns that the British security services are snooping on them.

Eurosceptics have started to hold important meetings face to face and to commit as little as possible to email because of the concerns that spies might be listening in on them.

The concerns are particularly shared by campaigners, MPs and even Eurosceptic ex-ministers because of their commitment to fighting the Chequers deal.

One Brexit source said they had found that their mobile phone calls regularly dropped off, clicks were heard on the line or scrambled noises were heard.

The source said: “We have great difficulties talking to each other on the phone. It is beyond bizarre the stuff that happens. We 100 per cent take precautions, as many precautions as we can.”

Another source said that at least one senior Brexiteer “did not discuss anything that sensitive over the telephone” when they were in Government.

Brexiteer MPs have discussed in private whether their conversations are being bugged, another source confirmed, although they had tried to brush off the concerns.

The claims were dismissed by a Government security source who said the Brexiteers were "paranoid".