Thousands of cross-Channel passengers are stranded after striking Calais ferry workers closed the French port and blocked Eurotunnel rail services.
The MyFerryLink workers walked out on Monday for the second time in a week over the sale of the company's ferries to rival firm DFDS Seaways.
Both Eurostar and Shuttle services were suspended due to fire on the tracks.
The tunnel reopened at 15:30 BST and Eurostar said "some services will run this afternoon".
However, Shuttle services are running with a three-hour waiting time at the Folkestone terminal.
Sailings between Dover and the French port are at a standstill. Services to Dunkirk, however, are operating as normal.
P&O Ferries said security at Calais had been "abandoned" leaving passengers "caught in the middle".
The Port of Dover said there was no known end date for the industrial action.
Operation Stack, where lorries use part of the M20 to queue for Channel crossings, has been implemented coastbound between junctions eight and nine, for Maidstone and Ashford, as a result of the latest action.
It has also been implemented on the London-bound carriageway.
Non-freight traffic travelling to Dover has been urged to use the A2 and M2.
Chris Childs from Hollingbourne in Kent said French strikers has caused major upheaval to the village next to the M20 for the second time in a week because of Operation Stack.
"Once again it feels like our village has been violated by French strikers," he said.
"We cannot get out of our village on to the A20 because of the traffic, likewise we cannot return easily."
Disruption for Eurostar passengers departing from Paris Gare Du Nord has been limited.
Jane Schaffer, from Birmingham, said her London-bound train left the station 30 minutes later than she intended.
"It was quite orderly, but there wasn't much information so you had to go find out," she said.
"There were large queues, but the train I'm on isn't even full."
Passengers hoping to travel to Calais by ferry have been urged to contact their ferry operators.
A strike which began on 23 June led to the suspension of Channel Tunnel services and saw hundreds of migrants try to board UK-bound lorries amid the chaos.
DFDS officially takes over the MyFerryLink service on Thursday.
The French ferry workers went on strike after Eurotunnel, which owns the ships, sold the cross-Channel service after a competition authority ruling to DFDS.
Calais security 'abandoned'
Helen Deeble, the chief executive of P&O Ferries, said: "Through no fault of their own, our passengers are caught in the middle of an industrial relations battle that has been caused by Eurotunnel.
"This has left thousands of holidaymakers and lorry drivers stranded without adequate facilities."
She said the company's staff had been "doing their best" to keep passengers supplied with food and water.
"Let me be clear, the buck stops with the French government. They have effectively abandoned any attempt to maintain security at the port of Calais," Ms Deeble said.
"When is the British government going to stand up to ensure that we can all get to mainland Europe safely and securely?"
She said the industrial action was putting thousands of jobs at risk.
A Eurotunnel spokesman said the French authorities needed to bring the situation at Calais under control.
"The real impact is on the British economy. We need to stop the strikes and get rid of migrants," he said.
On Monday, Transport Secretary Patrick McLoughlin chaired a meeting of the government's emergency committee Cobra.
A spokesman for the Department for Transport said the disruption caused by the industrial action to cross-Channel travel was completely unacceptable.
"We are working closely with the French Government and we want to see services return to normal as soon as possible," he said.
"Passengers, on both sides of the Channel, intending to travel through either Dover or Calais should check with their chosen operator before they travel."
Last week's disruption lasted 36 hours and saw blockades at the French port and the Channel Tunnel in Coquelles.
Several Eurostar trains were also forced to return to St Pancras, while in France migrants sought to exploit the port strike by attempting to climb onboard UK-bound lorries caught in slow-moving traffic.