Construction work has begun on a UK-funded wall near the so-called Jungle migrant camp in Calais.
Dubbed the "Great Wall of Calais" by some media, it is an attempt to prevent migrants from trying to stow away on trucks heading for Britain.
The 4m (13ft) barrier will run for 1km (0.6 miles) along both sides of the main road to the northern French port.
The UK government has said that while it provides money for security, French authorities choose how to spend it.
Local authorities in Calais say construction is expected to be completed by the end of the year.
The wall will pass within a few hundred metres of the sprawling migrant camp, which charities say now houses more than 10,000 people.
Home Secretary Amber Rudd previously told MPs the wall was "not a new initiative", but what mattered was making sure the French had the right amount of security to prevent "illegals" trying to get to the UK.
"We support the French with money to help them do that," she said. "It is up to them how they decide to secure their borders in Calais and around it."
Numerous fences have been built to protect the port, the Eurotunnel terminal and train tracks on the other side of Calais, and the BBC understands the wall will not replace any of those.
But Rod McKenzie, from the Road Haulage Association, previously told BBC Radio 4's The World at One that instead of stopping migrants from boarding trucks, it would just "move the problem further down the road".
Many of the migrants in northern France attempt to reach the UK by boarding lorries as they approach ports or the Channel Tunnel.
In August, BBC footage showed people-smugglers wielding sticks and dragging a felled tree on to the main port road to stop lorries and allow migrants to climb on board.
Earlier this month, French lorry drivers and farmers blockaded the main motorway route into Calais in a protest calling for the closure of the camp.