The fraud enquiry is centred mainly around fishing activity in the Mediterranean ports of Marseille and Sète.

Under-reporting of fish catches, and the illegal sharing of catches between boats of different countries, is a common problem in Europe, with the French believed to be amongst the worst culprits.

The EU have instituted proceedings against France and Italy for their failure to put in place proper procedures for control of tuna fishing.

It is also widely believed that the French maritime authorities have opened an enquiry into illegal fishing of tuna by French fishermen in Brittany, where once again it is believed the quota for 2007 was exceeded.

France has the largest tuna quota in Europe, of around 5,500 tons a year, accounting for 20% of the permitted quotas around Europe.

Government sources state that the current fraud may well exceed 1000 tons of tuna.

In September, the European Commission placed a ban on the fishing of blue fin tuna under the threat of extinction. By the time the ban was imposed the overall quota for capture for 2007 had already been reached.

Neither is the problem of illegal fishing restricted to France, or to tuna fish. Analysts believe that as much as 50% of the cod caught in the Atlantic is caught illegally, with Polish fishermen believed to be one of the worst offenders at under-reporting their catch.

One of the major problems is that, whilst the EU can determine quotas, individual countries are left to apply controls and penalties.