Cross-Channel travellers faced a further round of disruption after Eurotunnel reported "intensive migrant activity" at its French terminal.
The Channel Tunnel operator told passengers to expect delays of around two hours following the intrusion and due to a "very high level of traffic" leaving the UK.
Meanwhile, long queues of freight traffic continued to form on sections of the M20 in Kent where Operation Stack remains in place.
The Port of Dover said the motorway was closed coastbound between junctions 8 and 9 following the new problems at the Chunnel and high volumes of freight traffic.
But a port spokesman said it remained "open for business", with P&O Ferries and DFDS Seaways operating full services.
The fresh migrant problems in France come in the month that Home Secretary Theresa May announced a new secure zone would be created at Calais for UK-bound lorries amid the escalating migrant crisis across the Channel.
The area will be able to hold more than 200 vehicles, taking them off the open road at the French port where they have been targeted by thousands of migrants trying to reach Britain illegally.
The crisis in Calais has been rising over the summer, with more than 8,000 attempts by migrants to travel to Britain being intercepted in just three weeks between June 21 and July 11, Ms May said.
An estimated 5,000 migrants displaced from countries including Syria, Libya and Eritrea are believed to be in Calais.
The annual cost of products being written off because of contamination fears when migrants clamber on board trucks could reach £1 billion, it emerged this week.
Lorry drivers paid fines totalling £6.6 million for carrying illegal immigrants into Britain through Calais in the last year, official figures have shown.
The sum is over £2 million more than the £4.2 million paid to the Home Office in 2013/14.
French authorities have faced claims that they are not doing enough to safeguard lorry drivers from intimidation, violence and attempts by migrants to climb on.
Tim Waggott, the chief executive of the Port of Dover, has called on the UK Government to help ensure the £100 billion trade route is protected from disruption in the long term.
The Road Haulage Association (RHA) has suggested the deployment of the French military to boost security in Calais to protect truckers.
The Freight Transport Association (FTA) wrote to Prime Minister David Cameron this month, saying the situation in Calais was "deplorable" and conditions for drivers had reached "unacceptable levels".
Meanwhile, it emerged that a migrant drowned on Sunday night in Calais as he tried to break through a secure fence on to the Eurotunnel site.
On today's problems, a Eurotunnel spokesman said: "More migrants tried to get into the tunnel and tried to get on the trains there.
"We are obliged to shut down the tunnel when something like this occurs. There are around 5,000 (migrants) in and around Calais trying to get across the Channel and this has come on a very busy day."