A British law enforcement agency tackling organised crime on Thursday said social media companies were hindering efforts to combat people traffickers.

Rob Jones, director of threat leadership at the National Crime Agency, said police had asked for the closure of 1,200 accounts linked to "immigration crime" in the first five months of the year, but only 578 had been blocked.

"We were very certain when we made those referrals that there was a problem with those accounts," Jones told a parliamentary committee, a day after the government said more than 400 migrants crossed the Channel from France in small boats -- a record for a single day.

"To see that level of attrition... is challenging for us," he added. "They are applying their own decision-making and we have no traction over that."

A surge in numbers attempting the hazardous crossing of the world's busiest shipping lane is a key political issue in Britain, and the right-wing government has vowed to stop the practice.

Jones said traffickers were using encrypted communications via social media accounts to organise crossings of small boats ladened with migrants from northern France.

More than 5,600 migrants and asylum seekers have crossed the Channel in small vessels this year, according to Home Office figures. 

Prime Minister Boris Johnson has vowed to "address the rigidities in our laws that make this country... a target and a magnet for those who would exploit vulnerable people in this way".

The government has appointed a former Royal Marine, Dan O'Mahoney, to reduce the number of crossings. 

Also speaking Thursday, he said border authorities had been "exploring various different tactics", including efforts to counter "misinformation".

"There is a huge misinformation campaign going on with the facilitators, who tell them (migrants) the streets are paved with gold in the UK," he told MPs.

"As anyone knows who's travelled to France, it's a perfectly civilised country."

About half of those arriving on small boats in recent months have had their asylum requests assessed, according to the government.

Twenty percent of those have been granted, 10 percent rejected, and the rest have been judged ineligible, it added.

The arrivals have caused political tensions between London and Paris.

Britain's Conservative government has publicly pressured France to do more to stem the crossings. 

But French authorities insist they are doing all they can, and last month the mayor of the port city of Calais told Johnson he should "calm down".