French police have fired tear gas at hundreds of migrants trying to force their way onto lorries at Calais in a bid to reach Britain.
Local officers in the Pas-de-Calais region, where the port is located, said they had to call in reinforcements to deal with the asylum seekers.
"Some 300 to 400 migrants wanted to climb aboard the trucks and the CRS (riot police) had to use tear gas to disperse them," a police source told the AFP news agency.
The source also suggested that the line of lorries waiting to board ferries bound for Britain was longer than usual, and some migrants may have slipped into some of them unnoticed.
The northern French port of Calais has been struggling to stop illegal migrants trying to cross the English Channel to Britain, which is seen as having a favourable asylum policy despite recent measures to tighten security.
Last month, Home Secretary Theresa May reached an agreement with her French counterpart Bernard Cazeneuve to contribute up to £12m to a fund to help tackle the problem .
The deal, which is aimed at increasing co-operation between the British and French law enforcement agencies and strengthening security, came after Calais's mayor threatened to close the busy cross-Channel port.
Some 100 migrants were caught on film trying to storm their way onto a ferry to Britain , prompting the threat.
There has been a four-fold increase, to 1,500, in migrants at the French port town since the start of this year, according to officials. The migrants are mostly from Sudan and Eritrea.
Earlier this month hundreds of people took to the streets of Calais calling for more police officers amid the rise.