Ernest Moret, 29, is foreign rights manager for the French publisher Editions La Fabrique and the science fiction author Alain Damasio.

Mr Moret arrived in London on Monday evening by Eurostar for the London Book Fair. He had over 30 appointments planned with foreign publishers and had a return ticket to Paris booked for Friday 21 April.

On arrival at St Pancras Station, he was pulled aside by police officers acting under Schedule 7 of the Terrorism Act 2000 and detained for questioning – without a lawyer present – allegedly to determine whether he was engaged in terrorist acts or in possession of material for use in terrorism.

The police officers claimed that Mr Moret had participated in demonstrations in France, indicating complicity between French and British authorities.

He was told to surrender his phone and pass codes to the officers, with no justification or explanation offered. Yesterday morning, he was formally arrested and transferred to a police station, accused of obstruction because of his refusal to give up his pass codes.

In a statement, Éditions La Fabrique and Verso Books said: “We consider these actions to be outrageous and unjustifiable infringements of basic principles of the freedom of expression and an example of the abuse of anti-terrorism laws.

“We consider that this assault on the freedom of expression of a publisher is yet another manifestation of the slide towards repressive and authoritarian measures taken by the current French government in the face of widespread popular discontent and protest. It is crucial for all defenders of basic democratic values to express in the strongest terms that we find this intolerable and outrageous.”

Mr Moret has since been released from custody. A spokesman for Scotland Yard confirmed he had been arrested for obstructing its investigation – but refused to discuss what he had been questioned about.