Google is one of the world's most popular search engines

Companies pay to have their websites linked to particular terms on Google - one of the world's top search engines.

Thomson, Thomas Cook and First Choice were said to have acted after rival companies linked their websites to the names of the market leaders.

Google said such practices were stopped if trademarks were "used incorrectly".

"As stated in our Terms and Conditions, advertisers themselves are responsible for the keywords and ad text that they choose to use," Google said

"However, as a courtesy to trademark owners, Google will perform a limited investigation of reasonable complaints."

Google said if an advertiser uses a trademarked term as a "keyword trigger", those words would be taken out of its campaign.

Google searches return not just the most appropriate links as determined by the company's search engine, but also "sponsored links" at the top and right-hand side of the results page, with companies bidding for the right to have their website listed high up in these listings.

Thomson said Google was allowing businesses to "protect a brand name".

Trade publication Travel Weekly, which broke the story, reports that smaller agents who rely on the web to generate business have dubbed the Google policy "anti-competitive".