Lorry drivers were left queuing for five miles along the M20 in Kent as a trial of Brexit border controls were carried out at the Channel Tunnel.
Lorries on their way across the English channel were forced to wait in queues stretching back to Junction 11 on the M20 as they approached the Eurotunnel entrance outside Folkstone and the Port of Dover.
It is understood the delays were caused by French authorities conducting tests on border checking procedures at the Eurotunnel terminal in Folkstone.
A spokesman for Getlink, which operates the Channel Tunnel, said: “The backlog (of traffic) built up this morning during tests conducted by the authorities in preparation for Brexit.”
The spokesman said on Tuesday afternoon that traffic was now flowing “smoothly” through the terminal and services were operating as normal.
The queues have been seen as a glimpse of the difficulties ahead if the UK and EU fail to negotiate a deal by the end of Brexit transition period, which ends on December 31.
Both sides are continuing with negotiations however differences still remain over issues such as UK fishing rights and level playing field provisions.
The exercise is one of further intermittent border tests to be carried out before December 31.
Photographs showed dozens of freight lorries queued along the M20.
Highways England said in a statement: “The Port of Dover has informed us that new trial software for border checks has been identified as causing the delays, meaning 100% checks are underway.”
It said the test took place between 6am and 3pm on Tuesday and was carried out by Police Aux Frontieres, with tests also conducted at the Port of Dover.
Highways England tweeted: “M20 coastbound at J11 Westenhanger – the entry slip is now closed due to heavy congestion in the area due to delays at Eurotunnel.
“Due to the heavy congestion with HGV’s at this location, for safety reasons we have also closed the exit slip at J11 Westenhanger due to the amount of lorry’s parking up along the hard shoulder all the way back to exit slip.”
Downing Street has said will phase in these checks on the British side over the coming six weeks.
One source told the Guardian that delays were caused by a lack of staff on the French side and expect problems to ease with proper resourcing of the French border force.
Tuesday’s Brexit tests come a day after the UK government confirmed it would be shipping portaloos to the motorways as a “reactive” plan when long queues occur. It decided against installing them on a longer term basis because of the risk of drivers needing a convenience break stopping to use them when traffic is flowing at normal levels.