Up to a thousand migrants and refugees living in part of a camp on the outskirts of of Calais known as the Jungle have been told they must leave in the next three days or face eviction.

Under an expulsion order issued by the state authority in the French port, 800-1,000 people have been told to remove their makeshift homes and possessions by 8pm local time on Tuesday or be forcibly removed by police. Local aid workers say this figure could be considerably higher.

The provisional structures that have been built in the area, including shops, cafes, churches and mosques, will all be razed as part of efforts to clear regions of the camp next to a motorway leading to the port, where there have been clashes with police.

Plans to move people to heated shipping containers elsewhere in the camp and centres around France were announced by Fabienne Buccio, the top official in France’s northern Pas-de-Calais region, who said this month: “It’s time to tell the migrants of Calais who live in undignified conditions, and give Calais an image that isn’t dignified either, that we have a solution for each of you.”

But the migrants, many of whom are Syrian or Iraqi refugees, have previously said they would resist the move, insisting they wanted to stay in their tents despite poor living conditions.

Eight associations working in the camp, including Doctors of the World, have warned that the alternative accommodation is not suitable. In a protest letter to the French interior minister, the association wrote that the move is “very far from answering the needs of the problems encountered”.

Pascal Froehly, who volunteers for the relief organisation Caritas, spoke of his concerns of heightened tensions being created if bulldozers move in to level the site. He said: “I find it extremely annoying and unfair to move these people away from what they have created, including churches, shops and restaurants.”

Froehly added that the plans to move migrants to heated containers elsewhere in the camp offered them no chance to socialise. “It’s just a bed for them,” he said.

But the mayor of Calais, Natacha Bouchart, said dismantling the camp would keep migrants and refugees away from activists bent on causing disruption. She said it was a “sensitive situation” that required “necessary firmness”, and that the conditions endured at the Jungle were “unworthy of human nature”.

About 4,000 migrants live in the Jungle, many of whom are hoping to travel to Britain. A census carried out by the charity Help Refugees last week counted 440 children living in this particular section of the camp, 291 of whom were there without their parents.

On Thursday, some of Britain’s most prominent actors and writers, including Jude Law, Benedict Cumberbatch and Idris Elba, signed an open letter to David Cameron urging him to allow children stuck in the camp to enter the UK.

The letter asked the prime minister “to persuade the French authorities that the decision to destroy further parts of the camp in Calais is postponed until all the minors currently residing there are either given full child protection within the French system or enabled to reunite with their loved ones in Britain”.

It added: “This is a humanitarian crisis that needs to be acknowledged as such, and it is imperative that we do everything we can to help these innocent and highly vulnerable refugees, especially the minors, as swiftly as is humanly possible.”

Law, who visited the camp himself, said: “They are desperate. One little boy grabbed me and pleaded with me, that the Jungle was not a good place, and he didn’t want to be there.”

Last month, protesters staged a die-in at St Pancras station in London against plans to clear the area of the Jungle. Meanwhile, Eurotunnel has asked the British and French governments to reimburse it £22m for lost revenue during the cross-Channel migration crisis.

The Channel tunnel operator faced heavy disruption to its services last summer as migrants based at the Jungle made repeated attempts to cross to Britain.