Two men have died attempting a small boat crossing of the English Channel as the French authorities urged migrants not to risk the journey.

French authorities said the men, one thought to be in his 30s and the other in his 40s, died after leaving the coast at Berck on Sunday night.

Three other people among the 90 on board the vessel also needed treatment after prolonged contact with fuel in the boat, the local maritime prefecture said.

According to a translation of the prefecture’s statement, a separate incident on Sunday morning saw a boat containing around 80 migrants run aground at Camiers.

Most of the migrants waded back to the beach but 11 people were rescued suffering from hypothermia.

The Maritime Prefecture of the English Channel and the North Sea warned that the “flimsy inflatable boats used for such crossings are unsuitable and particularly vulnerable” to the conditions in the busy sea lanes between France and the UK.

“The maritime prefecture therefore calls on anyone planning to cross the Channel illegally to abandon their plan, regardless of the apparent weather and navigation conditions,” the statement said.

A spokesman for the Pas-de-Calais prefecture said: “An investigation has been opened by the Boulogne-sur-Mer public prosecutor’s office to determine the circumstances surrounding the deaths.

“François-Xavier Lauch, Prefect of Pas-de-Calais, thanks all the services involved, particularly the firefighters, for their intervention and commitment during this operation.”

The deaths came as police arrested 12 people following protests after a group of migrants were brought ashore in Hampshire.

A number of people were seen arriving at a marina in Gosport, having been transported aboard two RNLI boats late on Sunday evening.

Their dinghy was being shadowed by French rescue vessels until it was met by the RNLI boats in English waters, according to ship tracking data.

The Home Office said 149 migrants were believed to be on board the boat.

The vessel is the latest to use a route between Normandy and Hampshire rather than the shorter but more heavily policed crossing from the Calais region.

Twelve people were arrested for offences including assaulting a police officer following the Gosport protest.

Hampshire Police said around 350 people left Haslar Road near the marina at around 2am on Monday after the migrants were brought ashore by coastguard, lifeboat and border security staff.

The force imposed conditions under the Public Order Act as well as powers prohibiting the wearing of face coverings.

The Gosport arrivals were among 418 migrants who came to the UK on Sunday in five separate boats, taking the total for the year to 19,869.

But Downing Street insisted “real progress” had been made in tackling small boat crossings.

The Prime Minister’s official spokesman said: “Crossings are down significantly – 44% on last year – and this summer saw the lowest number of small boat arrivals in six years.

“We’ve removed more than 80,000 people with no right to be here since the election, an increase of 52%.”