Calais mayor Natacha Bouchart on Tuesday threatened to shut down the port unless Britain helps solve the problem of the hundreds of immigrants turning up there in a bid to sneak across the Channel.
"I could take the decision to block the port... I could bring pressure to bear," Bouchart told reporters in Paris after meeting Interior Minister Bernard Cazeneuve.
"It would be illegal," she recognised, "but today I want to make a firm statement to the British."
The mayor took issue with British immigration policy which, she complained, "is "considered as an Eldorado" by immigrants.
She also reproached London for demanding security is boosted at the Calais port without participating enough in financing the project, which, she said, cost €10 million ($13 million) a year.
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 financially with security at the port, a ministerial source said.
Bouchart and Cazeneuve also agreed on opening a day centre for immigrants in Calais, many of them from Africa, and a night shelter for women and children.
There are around 1,300 immigrants in the northern French port.
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 them in 1999 but rapidly became overcrowded, holding 2,000 people before it was closed in 2002, rather than the 800 it was built for.
Hundreds of would-be immigrants die in dangerous Mediterranean crossings every year, while others are detained by Italian police once they reach the southern EU member's territorial waters or the islands of Sicily or Lampedusa.
Clashes regularly break out between the immigrants in Calais.