Mr Lawton, who was on the train to Paris as part of a journey to Poland, said: "We are now in what seems to be a makeshift medical centre in Calais."

"We were evacuated off the train inside the tunnel and given gas masks.

"Then we were taken to an airtight concrete room within the tunnel and had to wait for two hours.

"We were then taken by coach to Calais. We still don't know what's happening."

Ben Lawton, in Calais, told the BBC how he was evacuated from his train inside the Channel Tunnel and was then given a gas mask because of the fumes.

For passengers facing disruption, Eurostar's customer care number is 03432 186 186, or +44 1233 617 575 for people outside the UK.

The Independent's travel editor, Simon Calder, said it was going to be messy and difficult for people trying to make alternative travel arrangements.

He added: "There aren't that many flights between London and Paris and Brussels on a Saturday afternoon or a Sunday morning. People will be trying to re-book for tomorrow.

"Motorists who were going on the tunnel from Calais to Folkestone will, of course, be able to just switch to one of the ferries which are operating. I understand that they do still have room."

He added: "We were stuck outside the Channel Tunnel for about one hour with no information at all, other than smoke had been detected and then we were told we would have to turn back to London.

"An hour later we're currently somewhere near Ebbsfleet with no more information other that what I have seen and read online."

John Hope was on a train to Paris to celebrate his girlfriend's birthday.

"We've had such a terrible experience this morning on the Eurostar to Paris," he told the BBC.

Mr Chatterton said crowds at St Pancras were "pretty calm" and said he had been told the tunnel was closed "indefinitely".

Kent Police said passengers should "expect significant delays whilst the vehicle is being recovered and fumes are cleared from the tunnels".

John Chatterton, who was travelling from London to Paris when his train was turned back, told the BBC the queue at St Pancras station was already 300 or 400 deep.

"I was on a train travelling from London to Paris for work.

"The train got stopped at Ashford and then we were told that we would be sent back to St Pancras.

"We've been told the tunnel will be closed all day and that there won't be any more trains today.

"We've been told we have to try and book again, possibly for tomorrow."

Image source, Johnny Chatterton

Queues have already formed at London's St Pancras station - a Eurostar terminal.

Tunnel operator Eurotunnel said in a statement that a CO2 alarm went off just before midday on Saturday.

The official Twitter account for Eurotunnel Le Shuttle's customer care team says, external: "Passenger and Freight services are currently suspended in both directions. We are awaiting further info from our teams in the Tunnel."

A Eurostar statement, external advised people planning to travel today to "postpone your journey and not to come to the station".

He said: "The investigation into the two alarms is going on at the moment so as soon we know more we'll let [everyone] know."

Eurotunnel spokesman John O'Keefe said inspectors were sent into the tunnel after two smoke alarms had been triggered - thought to be around midday.

There were no reported injuries and the lorry is being recovered, a Kent Police spokesman said.