Anti-nuke activists demanded an immediate suspension of Britain's nuclear submarine patrol yesterday after MPs heard of a radiation leak - over two years after the fact.

Defence Secretary Philip Hammond sought to "reassure" the Commons yesterday after admitting that engineers detected radiation spilling into the reactor coolant of the Trident fleet's Vanguard vessel in January 2012.

"These low levels of radioactivity are a normal product of a nuclear reaction that takes place within the fuel but they would not normally enter the cooling water," he said.

"This water is contained within the sealed reactor circuit and I can reassure the House there has been no detectable radiation leak from that sealed circuit."

Engineers failed to find the cause of the leak but had detected no further radiation leaks since the reactor was returned to use in November 2012.

But the Scottish Campaign for Nuclear Disarmament's John Ainslie described the news as "appalling."

Mr Ainslie said the submarines' continued round-the-clock operation was putting lives at risk.

"Trident and Astute submarines should all be kept in port until the cause of the fuel cladding failure is fully understood," he said.

The ageing fleet docked at Scotland's Faslane naval base is due for a £65bn replacement scheme before 2016 in order to remain functional, according to the Ministry of Defence.

But anti-war and anti-austerity movements have pressed Westminster to abandon the project instead, with full-cost estimates of up to £100bn.

The Scottish government has vowed to eject the fleet following a vote for independence in September - which experts say would leave the programme without an acceptable base of operations anywhere in Britain.