Ultimatum by Frank Gardner

Deep within a cave system at a military complex south-east of Tehran, hidden from the prying eyes of Western satellites, scientists are working round-the-clock on a banned device. Acting on the orders of a renegade cell within the ultra-conservative Iranian Revolutionary Guard Corps, their intention is to propel Iran into the elite club of nuclear-armed nations - and seal its domination of the Middle East.

Britain's intelligence agencies know something is up. They have someone on the inside ready to hand over information but first, he must get out of Iran. Crossing the mountainous border into Armenia, he was due to rendezvous with MI6 agent Luke Carlton at a remote, snowbound monastery. But the mission goes bloodily wrong. Luke needs to be extracted. And fast.

On the back foot after that fiasco, MI6 is presented with an opportunity to recruit someone with unique access to the leader of the IRGC hardliners. And Luke is chosen to reel them in. Going into Iran undercover is dangerous enough, but then a senior British government minister on an official visit is kidnapped, his close protection officer killed and Britain is presented with a terrifying ultimatum. With the clock ticking, Luke Carlton races to head off a catastrophic confrontation in the Gulf…