EU fishery ministers agreed Wednesday how to implement a ban on discards -- the wasteful practice of dumping unwanted fish overboard -- but failed to satisfy environmental groups.

After talks Tuesday ran overnight, ministers reached a "general approach" on the discard ban adopted in June last year as part of an overhaul of fisheries policy to put it on a more sustainable basis.

The key principle is that all fish caught will be landed, with none discarded, but fisherman will be granted leeway on existing quotas so that they can adjust over time to the new approach.

Under the accord, there will be a 9.0 percent exemption from quotas for the first two years, falling to 8.0 percent in the next two and finally 7.0 percent while some quotas may be transferable.

"Theses exemptions apply under strict conditions, in particular that all such catches should be fully recorded," a statement after the meeting said.

Ministers also agreed that some of the fish landed that otherwise would have been discarded could be used for charitable purposes.

The changes will be introduced from January 1, 2014 and be phased in through to 2019 depending on the fish species and the sea areas.

Environmental groups attacked the accord, saying the quota exemptions undercut the discard ban.

"France and Spain led a group of countries into dismantling a hard earned discard ban that was overwhelmingly passed through the European Parliament last month," the Oceana group said in a statement.

After agreeing the discard ban last year, talks on implementing it proved technically very difficult.

Small, young fish, for example, could be removed by changing net sizes but multi-species fisheries proved much more difficult since there was no obvious technical answer to prevent unwanted fish being taken.

In this case, any accord had to resolve what to do with the unwanted catch.

It has been estimated that up to a quarter of all fish caught in the EU is discarded, a huge problem at a time when most stocks are under pressure from over-fishing.