LONDON — A British journalist from the Guardian newspaper has been expelled from Russia after reporting claims in US diplomatic cables that the country had become a "mafia state", the paper said Monday.

Luke Harding, the daily's Moscow correspondent, flew back to the Russian capital at the weekend after two months in London reporting on the contents of the US cables, given to his paper by whistle-blowing website WikiLeaks.

But he was refused entry when his passport was checked on arrival and, after 45 minutes in an airport cell, was sent back to Britain on the next available plane, according to the Guardian.

He was given no specific reason for the decision, said the paper.

"I didn't go out to Russia with any particular agenda and I'm sad to leave under these circumstances," Harding was cited as saying in the Guardian.

"But I do not think journalists can accept self-censorship."

Alan Rusbridger, Guardian editor-in-chief, described the expulsion as "a very troubling development."

"It is worrying that the Russian government should now kick out reporters of whom they disapprove," he said.

Foreign Secretary William Hague has telephoned the Russian foreign affairs ministry to seek an explanation for Harding's expulsion but as yet has received no response, the Foreign Office said.

Harding's expulsion follows his reporting in December on assessments of modern Russia from the US cables, which listed a string of damaging allegations about the links between top officials, oligarchs and organized crime.

A Spanish prosecutor was quoted describing Russia as a "mafia state", while a top US official was cited questioning whether Russian Prime Minister Vladimir Putin knew beforehand about a plot to kill dissident Alexander Litvinenko.

Harding also coauthored a book, "WikiLeaks: Inside Julian Assange's War on Secrecy", lifting the lid on the paper's publication of the confidential documents.