Facebook’s use of a public relations firm which sought to smear George Soros should be investigated by the US Congress, according to a senior official from the billionaire tycoon's Open Society Foundation.

Patrick Gaspard, the president of Mr Soros’ Open Society Foundations charity, tweeted on Wednesday evening after Facebook admitted that it had asked Definers, a public relations firm, to conduct opposition research on its critics.

Facebook published an internal memo written by its current head of public policy, Elliot Schrage, in which he took responsibility for hiring Definers.

Mr Schrage wrote that he had told Definers to conduct opposition research. “I believe it would be irresponsible and unprofessional for us not to understand the backgrounds and potential conflicts of interest of our critics,” he wrote.

Definers sent a document to journalists which sought to link Mr Soros to several groups which had publicly opposed Facebook.