Using language normally used for dubious regimes, the Strasbourg-based Council of Europe asked British authorities to explain why they ordered the destruction of computer equipment held by the Guardian newspaper and the detention of a reporter's partner at Heathrow airport.
"These measures may have a potentially chilling effect on journalists' freedom of expression as guaranteed by the European Convention on Human Rights," council secretary general Thorbjoern Jagland said in an open letter to British Home Secretary Theresa May.
The Home Office declined to comment on the letter.
The Council of Europe runs the European Court of Human Rights, which enforces the rights code signed by the council's 47 member states.
Council spokesman Daniel Holtgen said the words "chilling effect" had previously been used in reference to situations in Turkey and Azerbaijan.
"Rarely has there been the case that we've expressed concern over a Western state," he said. "The bottom line is: we have to have the same standards."
Britain has been on the defensive since Sunday, when police used anti-terrorism powers to detain David Miranda, the partner of reporter Glenn Greenwald, and seize disks carrying sensitive journalistic material.
Mr Greenwald has reported on secret US domestic espionage using the work of whistleblower Edward Snowden.
The next day, Guardian editor Alan Rusbridger revealed that British spies were sent into the paper's basement to watch as disks containing leaked material were smashed.
The image of spies overseeing the destruction of journalists' hard drives rang alarm bells across Europe.
Mr Holtgen posed the question: what would have happened had a journalist's partner been detained in Moscow, or if a Russian newspaper had its hard drives smashed?
"You would have the Western press all over Russia," he said.