BRUSSELS (Reuters) - The European Commission on Tuesday warned Thailand, the third-largest seafood exporter in the world, for failing to crack down on illegal fishing, and threatened it with a trade ban if it did not take action.

The EU executive also lifted warnings on South Korea and the Philippines following the reforms of their legal systems to be able to tackle illegal fishing.

Since 2010 the EU - the world's biggest fish importer - has taken action against countries that do not follow international standards to prevent over-fishing, such as policing their waters for unlicenced fishing vessels and imposing penalties to deter illegal fishing.

Thailand was given a so-called "yellow card" for failing to certify the origin and legality of its fish exports to the EU.

It now has six months to implement a corrective plan to address the shortcomings in its monitoring, control and sanctioning systems, the Commission said in a statement.

"By using our market weight the EU is getting important players on board," said Karmenu Vella, Commissioner for Environment, Maritime Affairs and Fisheries.

"Both Korea and the Philippines have taken responsible action, amended their legal systems and switched to a proactive approach against illegal fishing".

The Commission will work with the Thai authorities to help them improve their legal standards.

The EU imported 145,907 tonnes of fish products worth 642 million euros (S$926 million) from Thailand last year, according to the Commission.

Thailand's foreign affairs ministry said in a statement Thailand was "disappointed" that the EU "chose to ignore the very earnest efforts of the past six months by the Royal Thai Government in addressing all issues which once were deemed the causes of IUU (illegal, unreported, unregulated) fishing".

However, it added that Thailand "will continue to cooperate with the EU to combat IUU activities and promote sustainable fishing".

Campaign groups such as the Pew Charitable Trusts, the Environmental Justice Foundation, Oceana and WWF welcomed the warning against Thailand.

"Thai authorities exert very little control over their fishing vessels, with many activities illegally damaging fish stocks and the marine environment," said Steve Trent, Executive Director of the Environmental Justice Foundation.

Between 11 and 26 million tonnes of fishing - at least 15 percent of global catches - are caught illegally each year and are worth between 8 billion euros and 19 billion euros, the Commission said.

Fish imports from Sri Lanka, Guinea and Cambodia are also currently banned pending progress from their governments.