Facebook and Google's duopoly in the digital advertising market could be under threat, after a number of leading British brands said they were reviewing their relationships with the companies amid mounting pressure to cut ties.

A number of high street names, such as Marks & Spencer and Unilever, are thought to be constantly scrutinising where they spend their marketing budget, although no companies said they had pulled their adverts from the sites.

MPs had on Thursday urged brands to re-allocate cash away from Facebook and Google, drawing advertising away from the sites, until those companies do more to stamp out terrorist activity. Currently, in the UK, Google and Facebook take around 55pc of every pound spent on digital advertising.

"The internet allows terrorist and extremist groups to create, post, copy and distribute extremist material, which can be made accessible to over a billion people in a matter of seconds," the report by the intelligence and security committee read.

Chairman Dominic Grieve said: "We recommend pressure is put on the communications services providers by the business community."