Paris (AFP) - Calais mayor Natacha Bouchart threatened to shut down the French port as around 100 migrants tried to storm their way onto a ferry to England on Wednesday.
The mayor said Britain was not doing enough to deter hundreds of migrants turning up in the town in the hope of getting across the English Channel.
Ramps to the ferries had to be raised to stop the migrants boarding the ships after 85 forced a gate and climbed over fences, overwhelming security staff.
A second attempt to force another entry into the port failed, as 150 migrants gathered around the entrances to the dock.
"Around 100 migrants succeeded in entering the port," port officials confirmed, who said the ramps were closed for between 10 and 15 minutes while police rounded the migrants up.
Ferry companies confirmed the "huge intrusion". P&O, who had a ferry at the quayside at the time, told AFP that it closed all its ship's doors, and that this sort of situation was "not usual" at the port, which has been a flashpoint for illegal immigration for more than a decade.
The mayor told reporters that she "could take the decision to block the port" after meeting French Interior Minister Bernard Cazeneuve in Paris on Tuesday.
"It would be illegal," Bouchart admitted, "but today I want to make a strong gesture towards the British."
The mayor took issue with British immigration policy which, she complained, is "considered as an Eldorado" by immigrants who have again flooded into the northern port over the summer.
Police said they are now between 1,200 and 1,300 migrants in the town, mostly from East Africa, and there have been a number of confrontations around the port in August.
Bouchart reproached London for demanding security be boosted at Calais without paying enough towards the 10 million euro ($13-million) annual cost.
Bouchart said she had not discussed the possibility of blocking the port with the interior minister, aware that he could not back such a measure.
"But I told him that I hoped he would have some strong negotiations with the British."
Cazeneuve, who was in London last Friday, had called on the British to help finance security at the port, a ministerial source said.
Bouchart and Cazeneuve also agreed on opening a day centre for immigrants in Calais and a night shelter for women and children.
Most are from Eritrea or Somalia and are hoping to reach England rather than seek asylum in France.
People fleeing war-torn Syria are adding to the rising numbers.
A Red Cross centre was opened for migrants in 1999 but rapidly became overcrowded, holding 2,000 people before it was closed in 2002. It had been built to hold 800.