A 250-vehicle convoy taking aid from Britain to the main refugee camp in Calais faces a last-minute ban by the French authorities at Dover, who are citing security concerns.

The French prefecture of Pas-de-Calais says those organising the convoy have called for a public demonstration and have invited organisations that support refugees from all over Europe.

Diane Abbott, the shadow secretary of state for international development and president of Stand Up To Racism, one of the groups organising the convoy, condemned the decision to block the group, made up of volunteers and others to deliver food, bedding and tents to the camp.

“It’s a shocking decision by the French authorities,” said Abbott, the MP for Hackney North and Stoke Newington. “This is a humanitarian convoy that wanted to bring help and support to people in awful conditions.

“It’s as if the French police think if they pretend the camp does not exist, you don’t have to help them. They don’t want attention drawn to the camp. I’m very sorry the French authorities are blocking it.”

Organisers of the convoy, which has been planned for months, deny that it is a demonstration. They say it was scheduled to leave the UK on 18 June in response to calls from refugee organisations for more volunteers and donations.

“This is unprecedented,” said Weyman Bennett of Stand Up To Racism. “This has been known about for months. We have been told that if we go, we are calling a demonstration and that when we get to Dover we will be turned back.

“It’s adding insult to injury leaving refugees in a perilous state in Calais and then denying them aid and solidarity on a false pretext.

“There is a huge need for aid and there have been a lot of negative comments about migrants. We wanted to show solidarity. There was a plan for a football match and face-painting for the kids.”

Bennett said volunteers had bought tickets in advance for the ferry and were planning to unload the aid at a warehouse on the outskirts of Calais before travelling to the camp for a rally. The convoy has been backed by Stop the War, the People’s Assembly and the Unite and Unison trade unions.

He said the convoywould travel to Dover whatever happens and demand that they get on the ferry with the tickets they have already paid for.

Clare Moseley of Care4Calais said: “It’s just really disappointing. The timing is really bad. Obviously we want the aid. It would have been good for the refugees to have that show of solidarity to see that people in England care.”

She said some of the camp’s residents had made kites and planned to fly them during the event.

“We’ve always been very clear from the start that we have no kind of protest or demonstration planned in France. We would have not agreed to it if there was going to be a demonstration.” She said that the camp had suffered from a decline in donations.

A statement from the head of the Calais prefecture, Fabienne Buccio, said the ban was necessary because of the security situation in France, which extended the state of emergency following the November terror attacks until after the Euro 2016 football tournament and the Tour de France cycle race. He also cited the double shooting of a police officer and her husband this week.

The prefecture said many people were expected to pass through the Pas-de-Calais area on 18 June, before the France v Switzerland game in Lille the next day. It added that a previous demonstration, jointly organised between refugee groups in Dover and Calais, had required police intervention.

A spokeswoman for the Metropolitan police said the force was aware of the convoy, which plans to meet at Whitehall before setting off for Dover. “We have appropriate policing plans in place,” she said.

Karen McVeigh and Lisa O’Carroll