Dramatic video footage shows thousands of litres of water spraying from the ceiling into one of the Dartford Crossing's tunnels.

But drivers will be relieved to hear the clip was filmed during a safety check and there was no danger to vehicles or their occupants.

The east tunnel, one of two that carries traffic northbound from Kent into Essex beneath the River Thames, was closed on Thursday evening and all vehicles were diverted through the west tunnel.

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Engineers then checked the tunnel's walls before testing the fire sprinkler system.

In the event of a fire being detected - either by a change in temperature or the presence of smoke – a "misting" system is triggered to keep the blaze suppressed until the fire brigade arrives.

Rather than flood the whole tunnel, the use of the sprinklers is limited to three 25-metre zones at a time - one where the fire is located and two on either side of the blaze.

When the misting system is running in all three zones, 2,500 litres of water pour out every minute.

Tanks at either end of the tunnel hold enough water to operate the system at full capacity for 30 minutes.

Steve Gooding, director of the RAC Foundation, said: “It’s good to know that the Dartford Crossing managers make every effort to avoid disruption to drivers while still carrying out the essential checks that ensure we can continue to cross the Thames in safety.

“The engineers do a sterling job in keeping one of the of the busiest stretches of road in the UK, if not Europe, up and running.

“The challenge will be to continue fitting all the necessary maintenance activity into an ever-shrinking window of opportunity as traffic levels continue their seemingly inexorable rise.”

The building of the crossing began in the late 1930s but was interrupted by the Second World War and resumed in the 1950s.

The west tunnel opened in 1963 and the east tunnel in 1980.

The QEII Bridge, which carries traffic in the opposite direction, from Essex into Kent, opened in 1991.

Combined, the two tunnels and bridge carry about 140,000 vehicles a day.