France is to send extra police officers to Calais, as migrants say they will keep trying to reach the UK despite the deaths of nine people since June, reports the BBC.
Migrants told the BBC going in groups of up to 400 gave them the best chance of getting into the Channel Tunnel.
Severe delays are affecting travellers in France and the UK. And Operation Stack is due to last all week, with 3,600 lorries queuing on the M20.
The home secretary said new fencing was urgently being installed in Calais.
Speaking after a meeting of the government's emergency Cobra committee, Theresa May said: "Crucially, what we are looking at now is improving security at the railhead at Coquelles, so we can ensure people are not trying to come through the tunnel."
Meanwhile, the Department for Transport announced it was relaxing rules that limit how long lorry drivers can drive for and how long they must rest.
French interior minister Bernard Cazeneuve said the extra 120 officers would be temporarily based in Calais and would help to secure the terminal area.
It comes after Eurotunnel said 2,000 migrants had tried to get into the tunnel terminal on Monday, and 1,500 had tried on Tuesday.