Ex-soldier's book Charlie One flying off shelves in Belfast
A book the Ministry of Defence had attempted to stop from hitting shop shelves is already on sale in Belfast.
Charlie One tells an insider account of missions carried out during the Troubles by the covert Force Research Unit.
The MoD has told Dublin-based publisher Merrion Press it knows the real identity of 'Sean Hartnett', the author's pseudonym, and that he will be arrested if he enters the United Kingdom.
He will also no longer receive his Army pension.
It has accused Mr Hartnett of breaching his special forces confidentiality agreement, disclosing classified operational details and breaching the Official Secrets Act. In a statement the MoD said: "While we would not comment on individual cases, the security of our armed forces is paramount.
"If we deem any publication to threaten that we request its withdrawal and take legal action."
Conor Graham, publisher at Merrion Press, said he had no plans to comply.
"We are seeking legal advice on the solicitors' letters and calls we have received from the Ministry of Defence, but we are not withdrawing the book," he said.
"The MoD has asked us to halt distribution. They claim the contents of the book pose a threat to Britain's national security.
"They have asked us not to distribute it until they can assess the implications.
"We don't believe there is an obligation on us to do this."
Bookshops in Belfast city centre have had Charlie One in stock for several weeks.
Waterstones said it had sold 80 copies and an Eason's staff member said it had also shifted a significant number of books.
Sean Hartnett grew up in Cork in the 1970s before joining the British Army in the 1990s, much to the surprise of his family, which had strong republican ties.
In 2001 he began serving in Northern Ireland with the secretive Force Research Unit, otherwise known as 'The Det'.
Hartnett claims to have been privy to "some of the biggest blunders by paramilitaries and British Army alike".
He was involved with the arrests of the 'Colombia Three' and chronicles the prevention of an assassination attempt by the UDA on Johnny 'Mad Dog' Adair.
Charlie One also claims to reveal new information about the 1987 Loughgall ambush, where eight IRA men and a civilian were shot dead by the SAS.
Soldiers opened fire on the gang as they were preparing to bomb the village police station.