Mr Szyszko acknowledged, however, that some big-name ads were still getting through.

Police suggest that if piracy site ads are not for "established brands", the platforms appear less legitimate
An anti-piracy police squad suggests it has caused a steep fall in the number of "big name" ads appearing on copyright-infringing sites.

Some adverts from legitimate companies are still being placed on piracy websites.
Brokers used by the leading media buying agencies are supposed to filter the sites they direct ads to, in order to screen out those on the blacklist.

"The strategy also does not tackle the underlying problem.

One expert had mixed feelings about the effort.

The figure is based on research carried out by Whitebullet - a firm that provides online intellectual property services.

The force says its effort has "almost entirely" stopped UK campaigns by the food/drink, car and property sectors being placed on the illegal web pages.

"Internet users demand ubiquitous access to quality content - for which they would gladly pay - so the creative industries would do better to focus their efforts on making their catalogues available on innovative internet-based distribution channels, like Spotify and Netflix, or invest in creating new user friendly services online.".

"However, if a few [advertising] sectors do indeed pull out, others will automatically take their place, and the effect on the website's income may be negligible.

Hundreds of sites now appear on the list, which has not been made public. Owners are given a chance to remove illegal material before their platforms are included.

"It shows copyright enforcement activities is no longer focused on violating internet user's civil and fundamental rights through monitoring all internet traffic, censorship, and shady political deals, as was the case just a few years ago," said Bendert Zevenbergen from the Oxford Internet Institute.

"You start getting less and less high value advertising - big brands, household name - and you start moving towards either no advertising or different formats of advertising that tend to pay less or are less credible from a consumer perspective - adult sites or sites advertising 'free games' that ultimately contain malware, are fraudulent or are generally just inappropriate.".

Operation Creative was launched by the City of London Police's Intellectual Property Crime Unit (Pipcu) in the summer of 2013.

One expert said the effect on the sites' earnings might be "negligible".

The firm's chief executive explained that ads are typically targeted at a local audience - so, a UK-based web user will normally see ads funded by a British campaign, even if the site in question is based elsewhere.

Mr Szyszko explained that sometimes brokers sub-contracted the work out to other third-parties who were less diligent, meaning the process was not foolproof.

"Holes can appear, and that's why you can still get the premium advertising appearing," he said.

It led to the creation of the Infringing Website List (IWL) - a database of online services that the police have confirmed provide access to unauthorised content.

Two years on, Pipcu says there has been a 73 % drop in advertising from the UK's "top ad spending companies" on the affected sites, which it suggests both reduces their income and removes their "look of legitimacy".

"The effort in the UK means that the advertising profile changes considerably," Peter Szyszko told the BBC.

It surveyed the ads placed on 17 sites that offer unauthorised access to TV shows, movies, music and games - both over a 12-week period between June and September 2013 and again between March and June 2015.

The scheme involves a blacklist, which is shared with ad brokers whose software determines what ads go where.