LONDON Defence Secretary Liam Fox has asked his permanent secretary to investigate claims that his links with a friend and unofficial adviser might have threatened national security.
Fox has called the claims "baseless" but Labour MPs said the inquiry was a panic measure.
Labour want to know why Adam Werritty, the best man at Fox's wedding, visited the minister 14 times in the last 16 months at the Ministry of Defence in central London, despite not being a ministry employee.
The heat on Fox rose when a photo emerged of Werritty with Fox in Sri Lanka while the minister was on an official visit there this summer. Fox told parliament last month that Werritty had not travelled with him on any official overseas trips.
Fox is on the right of Prime Minister David Cameron's Conservatives and is seen as a potential leadership challenger.
He risked Cameron's wrath last year when he fought hard against cuts to defence forces ordered as part of austerity measures to cut a record budget deficit.
A source at Cameron's Downing Street office backed Fox, saying he had taken "the right course of action" by asking for the internal inquiry.
Fox said his most senior official, Ursula Brennan, would "establish whether there has been any breach of national security or the ministerial code (of conduct)."
Labour defence spokesman Jim Murphy said in a statement: "The Secretary of State has ordered an inquiry into himself in order to prevent his top civil servant from forcing one upon him."
The Sri Lanka event he and Werritty had attended was a private gathering which his friend had organised, Fox told BBC radio.
Although Werritty is not part of Fox's team of officials, he is reported to have handed out embossed cards describing himself as "adviser" to the minister.
"I understand those cards are no longer used," Fox said. "I have made it very clear to him that it's unacceptable to carry a card saying that he is a personal adviser."
Werritty's meetings with Fox at the MoD concerned "entirely private matters, not to discuss MoD business," a defence spokesperson said. "At no time has he had access to any classified MoD documents or information."
The row comes as speculation rises over the position of Justice Secretary Ken Clarke following a high profile spat over human rights legislation with fellow cabinet minister, Home Secretary Theresa May.
Clarke had dismissed a claim by May that human rights laws -- unpopular with many on the Conservative right -- had blocked the deportation of a Bolivian man because he owned a cat.
Judicial officials had backed Clarke, who is on the left of the Conservative party, by saying the cat was irrelevant to the legal decision.
But Clarke later said he regretted describing as "laughable and childlike" the examples used by May to criticise human rights laws.
(Editing by Steve Addison)
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