UK Prime Minister Boris Johnson said action is needed with France to stop the increasing number of migrant boats crossing the English Channel as a Royal Air Force plane was sent to aid the task.

The number of migrants undertaking the perilous journey has risen during the more favorable summer conditions, with more than 4,000 having successfully crossed this year.

"We want to stop that, working with the French," said Johnson.

"There is a second thing we've got to do and that is to look at the legal framework that we have, that means that when people do get here, it is very, very difficult then to send them away again, even though they have come here illegally."

Junior health minister Helen Whately told Sky News that the illegal crossings were "unacceptable," and that the government was "taking action."

On Monday, the Ministry of Defence tweeted that it had sent a surveillance aircraft to support Border Force operations in the Channel.

The UK appointed a commander on Sunday to lead its response to the high number of illegal small boat crossings, that have included pregnant women and children on board. 

Interior minister Priti Patel said the UK was working to make the Channel route "unviable" and named Dan O'Mahoney, a former Royal Marine, as Britain's clandestine Channel threat commander, creating a new role to deal with the issue.

He will "urgently explore tougher action in France," a statement from the interior ministry said, referring to plans to intercept boats at sea and try to return them.

Last Saturday alone, 15 vessels made the journey to the UK, carrying 151 migrants.

The use of Royal Navy ships to stop the illegal crossings has been described as "dangerous" by a Calais politician, who added it would not make a difference in stopping the attempted crossings.

Pierre-Henri Dumont, the National Assembly member for Calais, told the BBC: "What is the British navy going to do if it sees a small boat? Is it going to shoot the boat? Is it going to enter French waters?

"It's a political measure to show some kind of muscle but technically speaking it won't change anything."

He said the French authorities needed to monitor about 300 miles of coastline if they were to stop migrants launching small boats from French shores.

"We are already trying to do whatever we can. We can't have a camera and police officer every 10 meters."