The Times has learnt that the confidentiality agreement was sent to all staff, irrespective of their seniority or how long they have worked for the party, without any warning or consultation at the end of last week.

It has caused uproar at Conservative Party headquarters as it forbids staff from divulging to any “third party” details of anything they see or hear at work even when they leave and move to new jobs.

The mere fact that the existence of the confidentiality document has been leaked will be the first test of whether it is legally binding or not.

The edict was sent by Giles Inglis-Jones, the new head of human resources, who came to the Conservative Party from British Nuclear Fuel. The giveaway is the initial BNFL at the bottom of the confidentialty form. The work of the state-owned BNFL, which supplies fuel to Britain’s nuclear stations, is highly sensitive.

The document has been privately dubbed Mr Cameron’s Official Secrets Act. One leading employment lawyer last night questioned whether the Tory high command would have any power to force employees to agree to the terms as it was being imposed retrospectively.

Michael Burd, a specialist in employment law at Lewis Silkin, said: “This is an interesting complexion on the new caring Conservative Party. It makes you wonder what they have to hide. The Conservative Party would have big problems if they were to say, ‘Unless you sign this you will be sacked.’They would certainly face unfair dismissal cases if they did get rid of people who refused to sign.”

One employee, who declined to be named, said: “Some of us are outraged and a lot of people are refusing to sign it. We all have contracts of employment already and there is a reference in them to confidentiality. This is a threat.”

The party appears to be retrospectively adopting the route of the Royal Family, which expects all senior courtiers to agree that they will not kiss and tell. Mr Cameron, who has unashamedly modelled his first few weeks as Opposition leader on Tony Blair, has also borrowed the idea from the Labour leader, whose early iron grip on his party was criticised as “control freakery”. All Labour Party employees sign a confidentiality agreement.

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