New security measures targeting thousands of migrants trying to get into the UK from the French port of Calais have been ordered by British prime minister David Cameron.

More sniffer dogs and Border Force search teams will be deployed to stop migrants breaching the fence surrounding the Euro tunnel at Calais and desperately trying to jump on trains to reach Britain, Mr Cameron said.

The announcement followed the promise of millions of dollars for improved fencing around the rail and tunnel complex.

Mr Cameron is under growing pressure to deter the migrants, many of whom have travelled from Africa and the Middle East, after repeated disruption to cross-Channel passenger and freight traffic, which has become front page news.

He made the announcement after calls from some politicians to mobilise the British army to reinforce border controls.

The French government had ordered extra police in an attempt to stop the migrants, but to little effect.

Re-elected in May, Mr Cameron promised to cut net annual migration to Britain to the tens of thousands — a pledge he failed to keep during his first 2010 to 2015 term in office when it hit a near record high of over 300,000 people per year.

The issue is a sensitive one for Mr Cameron as it plays into Britain's debate about Europe ahead of an EU membership referendum which he has promised to hold by the end of 2017 after trying to reshape his country's ties with the bloc.

Migrants have long gathered in Calais to try to board lorries and ferries to Britain.

But Eurotunnel, the firm that runs freight and passenger shuttles via the Channel Tunnel, said their numbers had now swelled to around 5,000 people from about 600 and that it was struggling to cope.

It said the migrants had also become better organised, mounting nightly attempts in large groups to storm the facilities.

Eurotunnel was sometimes forced to suspend its services because of such activity, causing disruption at what is one of the busiest times of the year for British holidaymakers.

The situation has caught the imagination of Britain's tabloid newspapers, becoming a political headache for Mr Cameron, who has spent the past week in south-east Asia on a trade mission while the crisis has flared.

He is under pressure to get tough on the migrants from many MPs in his ruling Conservative Party who said voters were fed-up with what they saw as lax border controls.

But he also had to contend with political rivals who were unhappy with the tone of the debate.

The opposition Labour Party criticised Mr Cameron for referring to the migrants as a "swarm of people".

It said the term was dehumanising and stirred public hostility against people often fleeing poverty and conflict.

Trucks remained backed up on the English side, as Operation Stack — which regulates delayed truck traffic — continued into the weekend.