The French Interior Ministry said the lorry, which is understood to have overturned on the train, was carrying the chemical phenol, a toxic product used by the pharmaceutical industry.
The incident resulted in "minor injuries" but no-one was seriously hurt, Eurotunnel officials said.
A train was sent to collect the people from the service tunnel and take them back to France.
The UK Foreign Office said seven of those evacuated from the tunnel were British.
The French state train company SNCF said services would not resume until Friday.
It is understood that no trains are stuck in the tunnel.
The tunnel carries Eurostar express trains between London, Paris and Brussels, as well as freight and passenger shuttles between Folkestone and Calais.
Eurostar said five of its trains were en route when the fire broke out, with 2,000 passengers affected. All trains have now returned to the stations where they began their journeys.
Traffic built up at the British end of the tunnel, with queues of lorries and cars tailing from the Eurotunnel terminal at Folkestone.
Kent police have closed junctions eight to nine of the coastbound section of the M20 and sections of the motorway have been used to queue lorries as part of Operation Stack.
The Highways Agency has told motorists to expect delays of up to an hour on the M20.
Motorists at the Channel Tunnel entrance were being given vouchers by staff to board ferries at Dover.
Meanwhile, passengers waiting to board trains at London's St Pancras International terminal were told to come back on Friday morning when they will be boarded onto trains on a first-come first-served basis. Long queues formed as passengers tried to get information.
Eurostar said anyone with a ticket for Friday would be able to travel if the trains were running, and some travellers said Eurostar had agreed to cover the cost of their overnight stays in London.
A spokesman said anyone who had been unable to use the tickets because of the fire would be given as much help as possible with continuing their journey.
There were similar scenes at Gare du Nord station in Paris, with hundreds of passengers stranded.
Eurolines, which is part of National Express, said it had capacity to take Eurostar passengers to Paris and Brussels by coach, via cross-Channel ferry services.
Channel Tunnel passengers on 'nightmare' journey
Clive Evans and Albert Cole were on board a Eurostar train to Brussels, with plans to stay with friends for the week, when their journey was terminated.
Mr Evans told BBC News website: "We got as far as Ashford and they said 'You're not going any further. You can either get off here and get the ferry to France' - God knows where the ferry goes from in Ashford - 'or you can go back to London.'"
He said: "We've just got back [to London]. It's total chaos. They say they can't put us up in hotels because there's too many of us. It's total disorder."
The Channel Tunnel has suffered several incidents since it opened to traffic in 1994 although only one - a fire in 1996 - caused injuries.
In August 2006, 34 people had to be led to safety after a fire broke out on a lorry being carried on a freight train.
Security exercises are staged in the Channel Tunnel by police, fire and ambulance services from both England and France to ensure preparedness for such incidents.
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