A new French migrant detention centre the UK has offered to help fund is facing a legal challenge that could hamper a £660m deal to tackle illegal crossings of the English Channel.
The lawsuit could delay the opening of the centre, which the UK has only agreed to contribute money towards once the facility near Dunkirk has opened.
The Home Office said funding for the centre from a £160m pot would be withdrawn if the deal was not delivering proven results within its first year.
The legal challenge risks hindering a key part of the agreement, which Home Secretary Shabana Mahmood said would help "restore order and control to our borders" when signing the deal in France last month.
Prime Minister Sir Keir Starmer is under pressure to curb the heightened levels of people attempting to cross the Channel in small boats in recent years and this agreement with France is a significant element of his response.
The new detention centre in the Loon-Plage area is under construction and the Home Office said it was expected to be operational by the end of this year.
While building work can continue during the legal challenge, French legal experts told the BBC the facility's building permit could be revoked if the lawsuit was successful.
But delay is more likely as legal challenges often slow down the process of opening migrant detention centres in France, rather than stopping them entirely.
"France has committed to building this detention centre," a UK government spokesperson said. "The UK will only pay when the work is completed."
The French government has not responded to the BBC's requests for comment.
The ongoing costs of building and running the new detention centre have not been revealed by the French government.
But a recent report by the Senate, the upper house of the French Parliament, said a standard 140-bed detention centre costs about €40m (£36m).
France's Ministry of the Interior was granted a permit to build a detention centre with the capacity to hold 140 people in July last year.
Flemish-Artois Coastal Environmental Defense Assembly, an environmental group known as ADELFA, challenged the decision in November last year, arguing the permit should be withdrawn because the facility violated local planning rules.
The challenge was rejected and ADELFA filed an appeal at the Administrative Court of Lille in February this year.
The group's lawyers argue the facility is located in an industrial zone where residential accommodation is not allowed under planning rules.
The appeal says the site is near industrial facilities, including a warehouse with ammonia refrigeration, which "creates significant health risks for occupants".
The lawsuit also accuses the ministry of breaching fire safety regulations and not posting its building permit in a place visible to the public, as the law requires.
Nicolas Fournier, the president of ADELFA, said although winning the lawsuit was "not certain", he was "still trying to hinder this process of building the detention centre with the appeal".
"Putting so many resources solely into repression, with ever more police, doesn't work," Fournier told the BBC.
"So we really need to find other solutions, because we can't continue to allow this risk of seeing people take to the sea in unacceptable, deplorable conditions that endanger them."
The appeal is not suspensive, meaning it does not immediately block the construction of the detention centre while the legal process is ongoing.
France committed to build the detention centre in 2023, under a previous deal with the Conservative government when Rishi Sunak was prime minister.
The 2023 agreement said the centre would "contribute significantly to improve the number of returns and prevent the recurrence of crossing attempts".
But progress on the centre has been slow.
Under the new deal, signed by Mahmood, about £160m of the UK's funding has been linked to results on stopping Channel crossings for the first time.
Under the new deal, France has agreed to target and detain migrants seeking to board small boats on its beaches.
Some will be held at the new detention centre, which is expected to be staffed by more than 200 French officers, before the migrants are deported back to their home countries or other EU nations they have passed through.
Once built, the centre would focus on removing migrants from Eritrea, Afghanistan, Iran, Sudan, Somalia, Ethiopia, Iraq, Syria, Vietnam and Yemen, the Home Office said.
These places represent the top 10 countries of origin of people who attempted crossings in small boats last year.
The approach will be trialled using existing capacity at a nearby removal centre in Coquelles from May this year, until construction of the facility near Dunkirk is completed.
Dr Mihnea Cuibus, a researcher at The Migration Observatory, said there were many barriers to effectively scaling up migrant removals from the detention centre.
He said the success of the centre was a "potentially contentious issue" in UK-France relations.
Francois Benchendikh, a senior lecturer in public law at the Sciences Po Lille university, said the debate in court will focus on whether the Dunkirk centre is considered to be residential accommodation or not.
The presence of ammonia near the site could lead to the annulment of the building permit, he added.
Alice Darson, an urban planning lawyer from Paris, said the detention centre should not violate planning regulations as it falls under the category of "facilities of collective interest and public service".
But, she added, the building permit could be cancelled if the fire safety authorities were not properly consulted.
"In that case, the building could be demolished," she told the BBC.
