Liam Fox, the defence secretary, is refusing to accept anything other than a minor delay in replacing Britain's Trident nuclear deterrent, say government sources.

In a move which threatens a row with Nick Clegg and the Treasury, Fox is telling cabinet colleagues that replacing the four Vanguard nuclear submarines can only be delayed by four to six months.

His intervention shows that the deputy prime minister is struggling to influence an intense Whitehall battle over the replacement of Trident.

Clegg, who argued during the election against a "like-for-like replacement", gave the impression today that the Liberal Democrats were still making the case in Whitehall for "alternatives".

Asked on the BBC1's Andrew Marr Show whether a replacement for Trident might not be agreed in this parliament, he said: "Everything is possible for the simple reason that the Ministry of Defence would need to fund this replacement from its own budget ... I don't think people would understand why we would exempt Trident from the same financial pressures when people are having some of their benefits qualified in different ways."

His remarks were dismissed in Whitehall tonight by sources who said that the government had ruled out alternatives such as a land based silos or nuclear- equipped long-range aircraft. "We haven't yet found an MP who would be happy to have nuclear missiles based in their constituency," one source said.

Ministers have agreed that Britain's new nuclear deterrent should be similar to the current "continuous at sea deterrent", which consists of four submarines carrying atomic warheads. One submarine is always on patrol and able to launch a strike on any target anywhere in the world; a second is on standby ready to be launched at a moment's notice should the first be attacked or suffer an accident; the other two submarines are, at any one time, in different stages of being serviced. "Trident will be replaced with a submarine based system," the Whitehall source said. "The coalition agreement allows for an exploration of alternatives. It is not clear what the alternatives are. But we are carrying out a value for money assessment, as set out in the coalition agreement."

This assessment is looking at two key options for cutting costs:

• Reducing the new fleet of submarines from four to three. It is understood this has been rejected by Fox on the grounds that three submarines would not guarantee the continuous deterrent.

• Delaying the building of new submarines by as much as five years. Ministers were said to be interested in a paper by Malcolm Chalmers, of the Royal United Services Institute, suggesting such a delay. The oldest Vanguard submarine is due to be taken out of service in 2022, with the second due to follow two years later.

It is understood Fox is insisting that building the new submarines can be delayed by no more than four to six months. "A delay of four to six months is not an issue," the source said. "A two to five year delay is not on."

Fox is making clear that prolonging the life of the Vanguards would jeopardise the "continuous at sea deterrent" and would not represent value for money. "It would be very expensive to extend the lives of the boats for a few years," the source said.

But the Vanguards could be extended by a shorter period by slowing down the servicing of submarines and warheads. At the moment this has to take place quickly to ensure that one submarines is on patrol and a second is permanently on standby.

This proposal is being subjected to a "risk analysis" to see whether slowing down the servicing of the submarines and warheads would jeopardise the "continuous" element of the deterrent.

The initial costs of replacing Trident during the spending review for 2011-14, due to be announced by the chancellor, George Osborne, next month, is a relatively modest £700m. The main spending of around £2bn a year would kick in during the next spending period from 2015.