The Government has called on competition watchdogs to examine the stranglehold of Google and Facebook over the £14 bn digital advertising market amid concern that “opaque” practices deny publishers a fair share.

Jeremy Wright, the Culture Secretary, has written to the Competition and Markets Authority (CMA), urging it to study the sector for evidence of abuse of dominance by the Silicon Valley giants. It follows publication this week of a report on the sustainability of journalism by the economist Dame Frances Cairncross, who said Google and Facebook may be restricting competition and stifling innovation. Between them they had captured 54 pc of the digital advertising market.

Mr Wright backed Dame Frances’s findings and told MPs: “Online advertising now represents a growing part of the economy and forms an important revenue stream for many publishers.

“But this burgeoning market is largely opaque and extremely complex, and therefore it is at present impossible to know whether the revenue shares received by news publishers are fair.

“The purpose of this study would be to examine whether the online marketplace is operating effectively, and whether it enables or prevents fair competition.

“It is right that policymakers and regulators have an accurate understanding of how the market operates, and check that it is enabling fair competition.”