Facebook and Google have warned staff not to ask colleagues out on a date more than once if they are rebuffed the first time.

In a crackdown on workplace romances, staff have been told that if their advances are rebuffed, rules state they are not allowed to ask the same colleague out again.

It comes amid a rise in so-called 'love contracts' or 'relationships at work contracts' - which have become more common at leading U.S. companies to clarify complicated workplace employee dynamics.

Facebook and Google have warned staff not to ask colleagues out on a date more than once if they are rebuffed the first time. In a crackdown on workplace romances, staff have been told that if their advances are rebuffed, contract rules state they are not allowed to ask again

They are designed to prevent conflicts of interest, such as when a manager attempts to woo a more junior colleague.

Someone trying to set up a date who is told 'I'm busy that night' could be breaking their contract if they make a second attempt to chat up a workmate, American staff are told.

Heidi Swartz, Facebook's global head of employment law, told the Wall Street Journal that staff must disclose relationships which could lead to a conflict of interest.

However, London-based employment law experts said similar moves would be difficult to enforce in the UK because of the European Convention on Human Rights, and its right to privacy and family life.

Jonathan Maude, of law firm Vedder Price, said: 'In the U.S. they don't have the same concept of privacy.'

Mr Maude explained that he had dealt with one instance where a manager was almost sacked after dating and then breaking up with a junior member of the team.

It comes amid a rise in so-called 'love contracts' or 'relationships at work contracts' - which have become more common at leading U.S. companies to clarify complicated workplace employee dynamics (Facebook's London HQ)

He said: 'In that case it resulted in disciplinary proceedings - he wasn't fired but he was given a final written warning and was told that it was his last chance.

'What I advised there was that the manager should come clean. If he had, then it would have avoided the problems.'

Adding that he believed sending managers on courses about sexual harassment would be more effective than resorting to 'love contracts' in the workplace.