LILLE, France (AFP) - A man was found dead on Tuesday in the Channel Tunnel, the authorities in northern France said, in yet another example of the lengths taken by migrants desperate to reach Britain.

Vehicle shuttle services and Eurostar passenger trains were delayed as one of the six sections of the tunnel was closed for investigations, although it remained unclear how the migrant had died.

"Following the discovery of migrants on a freight shuttle, early this morning, the shuttle was immediately stopped," local authorities in the Pas-de-Calais region said in a written statement.

"During an investigation by border police, a deceased migrant was found in the tunnel," they added.

"For the moment we suppose he wanted to climb onto the shuttle, which he missed, and that he was hit by the train itself," said Mr Patricio Martin from the border police, referring to the shuttle train that takes road vehicles through the tunnel. The identity of the migrant was not known, but he was among some 40 people evacuated from the tunnel.

Although traffic was only briefly interrupted in the tunnel, the incident triggered three-hour delays on vehicle shuttle services coming from the British side and an hour from France, according to operators Eurotunnel.

Eurostar trains were also delayed.

Intrusions into the undersea tunnel have risen in recent weeks as increasingly desperate migrants seek new tactics to enter Britain.

On Friday night, about 150 people tried to enter the boarding area before being pushed back.

"Currently they are increasingly turning to the tunnel under the Channel, that is why we have security reinforcement at night and it is true that we have more intrusions than usual," said Mr Martin.

Thousands of migrants are camped out around the port in the northern city of Calais, in the hope of climbing aboard lorries travelling to Britain on ferries or entering the nearby Channel Tunnel.

The migrants, whose presence has long caused friction between London and Paris, sometimes go to dramatic lengths to smuggle themselves into Britain, and have even been recorded trying to swim across the Channel.

In recent days, traffic through the Channel Tunnel has been repeatedly disrupted due to protesting French sailors, as well as attempts by migrants to enter the tunnel.

Late last month, a migrant from Ethiopia died near the tunnel as he tried to get onto a train in the hope of getting to Britain.