Facebook and Google take around 55pc of every pound spent on digital advertising in the UK - AFP

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."

He said companies should follow Unilever's lead, which has threatened to pull its adverts off the sites in the past.

Unilever said it was "committed to working with key partners to ensure greater accountability for their platforms and the influence they yield in society", and would always look to work with those sites before pulling any ads.

The latest pressure from MPs follows a number of high-profile instances where adverts of major brands have appeared alongside child abuse videos.

Last year, for example, an investigation by The Times revealed that adverts for names such as Adidas and Hewlett-Packard had appeared next to YouTube videos of scantily clad children.

The revelations resulted in more than 250 brands suspending their YouTube ads. Some of those companies have since returned.

The ISBA, which represents advertisers, said: "It is beyond doubt that brands take their responsibilities in this area seriously, with action being taken by some to remove their advertising entirely from some platforms when they feel their concerns are not being addressed.

"However, action in concert would not be legal and it is up to each advertiser to make their own decisions...The recent proposals from Facebook for an independent content appeals board goes some way towards meeting ISBA’s calls for an independent oversight body, funded by industry."