The arrest and detention of a British journalist under anti-terror legislation has been condemned as likely having a “chilling effect on journalists in the UK and worldwide”.

Freelance foreign affairs reporter Richard Medhurst says he was arrested under Section 12 of Terrorism Act 2000 and told he was accused of “expressing an opinion or a belief that is supportive of a proscribed organisation”.

He says he was held for nearly 24 hours after being arrested on a plane at Heathrow Airport in London and does not yet know whether he will be charged with any offence.