Ministers will announce plans to buy the most advanced and expensive nuclear reactors for new Trident submarines this week in a move set to create tensions within the coalition government.

The decision has been delayed by months due to the sensitivities within the cabinet and concerns over costs, with Liberal Democrats keen to avoid progress on the issue. The party's manifesto had stated it was against a like-for-like replacement for the country's nuclear deterrent.

However, a source said a decision had now been made among the coalition partners in favour of the more expensive reactors and an announcement would not be delayed any further.

The decision is likely to add several billion pounds to the £20bn cost of replacing the UK's four Trident submarines, but the new reactors are understood to be safer. The source said: "It has all been agreed and the announcement will be made this week. Nothing pleases the Liberal Democrats on this subject but they have signed up on it, so it will happen."

The deputy prime minister, Nick Clegg, has been praised within his party for persuading the prime minister to delay the irreversible "main gate" decision on Trident, when binding contracts are signed with companies building the four submarines, until after the next general election in 2015.

The "initial gateway" decision on whether the new submarines are fitted with the existing type of reactors or new modern PWR3 reactors had also been delayed by eight months. It is understood that the Treasury, in particular, had been wary of committing to the more expensive reactors despite lobbying by the Ministry of Defence.

However, the former head of the Defence Nuclear Safety Regulator, Commodore Andrew McFarlane, recently wrote that the current reactors were "potentially vulnerable to a structural failure of the primary circuit".

He claimed a failure could cause a leak of "highly radioactive fission products" and "a significant risk to life to those in close proximity and a public safety hazard out to 1.5km from the submarine".

Labour MP John Woodcock said the government should explain to the public why there had been delays when the health and safety arguments were so compelling. He said: "The disorderly friction between the Tories and Lib Dems on such a crucial issue is bad for national security. The timetable on renewal is now needlessly tight – the government need to explain why they have delayed firing the starting pistol for so long."

The prime minister has always maintained that the UK needs to keep its nuclear deterrent, and replacing Trident was a Conservative manifesto pledge. However, the Liberal Democrats have been opposed. The coalition agreement stressed that Trident would be renewed but said the Lib Dems would continue to look for alternatives. "We will maintain Britain's nuclear deterrent, and have agreed that the renewal of Trident should be scrutinised to ensure value for money," the agreement states.

Danny Alexander, the Lib Dem chief secretary to the Treasury, has repeatedly insisted that he believes spending on Trident does not represent the best use of taxpayers' money.

Last week police arrested 10 anti-nuclear campaigners who blockaded the Faslane naval base on Gare Loch – the home of the UK's Trident fleet. The activists said their protest was a call for "the anti-Trident SNP government to fulfil its commitments and demand immediate disarmament of Trident and a global ban on nuclear weapons".