The idea that the cold war would never come back has been demolished by the crisis in Ukraine, said the Tory MP, Julian Lewis. What if Putin threatened one of the Baltic states, all of which are members of Nato?

Lewis, an inveterate defender of nuclear weapons, was speaking at a debate on the future of the UK's Trident nuclear missile fleet organised by the International Institute for Strategic Studies in London.

Britain needed a nuclear arsenal, with one Trident submarine continuously at sea as an "insurance policy against the unknown", and blackmail, said Lewis.

Would Britain have dared to respond to the 1982 invasion of the Falklands had General Galtieri's Argentina possessed nuclear weapons?, he asked a question which suggests that every potential aggressor should try and get their hands on nukes.

"You can never predict what's going to happen", Lord Alan West, a former head of the navy and security minister in the last Labour government, told the IISS meeting on Tuesday. "Some things are more important than cost", he added. Trident was was the "ultimate safeguard" in an "unpredictable and chaotic world".

The former Liberal Democrat defence minister, Sir Nick Harvey, argued there was "a logic" at the time of the "balance of terror" during the (actual) cold war when there was a need to be able to strike at a moment's notice. But there was no need now to have a nuclear sub always at sea. Britain should maintain a "nuclear capability", Harvey said, but not with a Trident submarine on patrol, 24/7.

Ah, Lewis responded, but we would not know whether the threat was imminent, and it would be too late to get a Trident sub ready in time. "Continuous at sea deterrence" or CASD, as it is called, was essential he implied.

"Deterrence does not depend on uncertainty but on certainty", Lewis continued. But we were always told during the cold war that the whole point of the UK's nuclear arsenal, alongside America's, was that it created uncertainty in the minds of a potential aggressor.

It is hard to imagine the circumstances when a British prime minister would be "certain" to order a nuclear strike and make the deterrent credible.

The foreign secretary, William Hague, has described the Ukraine crisis as "the most serious test of European security in the 21st century so far". Instead of ramping up the rhetoric, world leaders could concentrate on genuine confidence-building measures at next week's Nuclear Security Summit in The Hague.

After the IISS Trident debate, Harvey told the Guardian that instead of phasing out CASD, Britain had "sustained it at gargantuan effort and expense, waving it around on the high seas, pointing at no-one in particular and for no obvious practical utility".

Britain 's current posture on CASD was based on "an outdated and paranoid worldview. It is quite simply time to move down the nuclear ladder and end CASD patrols at the very least".

Trident was a drain on a vastly-shrunken defence budget, Harvey added. When capital spending on a successor Trident fleet reached its height, sometime between 2017-2030, a range of other vast defence projects would be competing for very limited funds: new JSF planes for the aircraft carriers, Type 26 frigates, and new kit for the army – amongst many others. Balancing all of this is virtually impossible: something will have to give.

"It is surely more important to participate in coalitions of the willing with our international partners than sustain the current level of deterrence. Maintaining Trident endangers [the special relationship with the US] so there is a choice to be made," Harvey said.

The US has expressed concern about Britain's ability in future of the UK - not to threaten countries with nuclear weapons, but to conduct conventional (and more credible) military operations.

For years, army chiefs, who now face a huge cut in the number of their soldiers to 82,000, have its lowest level since the Napoleonic wars, have questioned the relevance of Trident.

General Sir Peter Wall, the head of the British army, told the Chatham House thinktank last week: "We have only got to look to the tension in the Ukraine to see a situation that was not foreseen and is confounding our previous assumptions about stability across Europe," What a deterrent was, he said, was "having ready [land-based] forces sends strategic messages to potential adversaries..."

Well even that seems questionable with the west barely able to agree on limited economic sanctions on Russian individuals in protest against the decision to take Crimea away from Ukraine (or returning it to Russia depending on your point of view)

There are those who say that Putin has managed to reinvigorate Nato, give it a new purpose, as it phases out its operations in Afghanistan. That seems a bit scary.

Better, perhaps, to build up confidence-building measures, proposing - and guaranteeing - Ukraine's neutrality rather than provocatively holding out the possibility of its membership of Nato.