LONDON, June 5 (Reuters) - Google parent Alphabet

must face a mass lawsuit which accuses it of abusing

its dominance in the online advertising market, London's

Competition Appeal Tribunal (CAT) ruled on Wednesday.

The lawsuit seeks damages of up to 13.6 billion pounds

($17.4 billion) on behalf of publishers of websites and apps

based in the United Kingdom, who say they have suffered losses

due to Google's allegedly anticompetitive behaviour.

Google last month urged the CAT to block the case, which it

argued was incoherent, but the tribunal said in a written ruling

that it would certify the case to proceed towards a trial.