French police have banned “all demonstrations related to migrants” in Calais on Saturday 18 June, the day a huge aid convoy was scheduled to head from the UK to the port town where thousands of refugees and migrants are living in squalid conditions.

A letter sent to the British police by their French counterparts on Wednesday and seen by Middle East Eye warns that the planned aid convoy – which includes 250 vehicles carrying aid for people living in and around Calais – could constitute a “grave threat to public order”.

The letter notes that, in October 2015, “British militants” supportive of migrants staged a sit-in within the port area and marched through the centre of Calais, leading to clashes with the French police.

“In these circumstances, and considering the large numbers expected [to take part in Saturday’s convoy], there are serious reasons to think that the demonstration on 18 June risks becoming a grave threat to public order.”

The letter – signed by the head of police policy in Calais – also notes that, due to the ongoing Euro 2016 football tournament being held in France, there will not be enough police on hand on Saturday to ensure that the convoy’s visit passes off peacefully.

The planned convoy has been organised by eight UK-based organisations, and is headed up by a donated articulated truck carrying dozens of tonnes of aid.

Chris Nineham, one of the activists organising the convoy, told Middle East Eye that French police “have no reason whatsoever to think there’ll be a demonstration” during the event, which will take place outside the main migrant camp and include a small number of speakers.

“This is essentially a humanitarian initiative – the convoy is partly a gesture of solidarity, but it’s mostly about taking aid to people in desperate need.

“This is very bad news for the people in Calais, as it will limit the provisions they have. For the French police to be blocking this is extraordinary.”

The convoy’s organisers – which include Stop the War, Stand Up To Racism and the Muslim Association of Britain – say they still plan to set off from Whitehall, the London street considered the headquarters of the UK government, towards Calais on Saturday.

They have written a petition calling on British Prime Minister David Cameron to put pressure on French authorities to allow the aid to cross.

British activists working to take aid to Calais have previously complained that police in both the UK and France have hampered their efforts.

Some say they have been stopped by the British police and questioned under Schedule 7, an article of the Terrorism Act that gives authorities the power to stop, search and hold people at international transport hubs if they are suspected of involvement in terrorism or any other criminal activity.

One, a co-founder of the group London2Calais, recently told Middle East Eye that he had been banned from France because of his group’s aid activities.

Activists and aid workers inside France are also subject to laws that penalise people giving support to anyone who “irregularly enters or stays” in the country.