Dr Julian Lewis: In the past few months we have had several opportunities to debate nuclear deterrence. The hon. Members for Islington North (Jeremy Corbyn) and for Brighton, Pavilion (Caroline Lucas) and I, from our respectively opposite sides of the argument, successfully procured a debate on 17 January. Strangely enough, I did not hear many of these Liberal Democrat midway positions articulated on that occasion. The hon. Member for Islington North then secured a debate in Westminster Hall on nuclear deterrence and the non-proliferation treaty on 20 June, and I seem to remember that there were no Liberal Democrat contributions to that debate at all.

I think that it is possible to make a principled and coherent case either that we should have an effective and continuous nuclear deterrent or that we should not, but one cannot make a sensible case for having a part-time deterrent. I have looked at the report in some detail and will pick out a couple of elements that I regard as particularly significant. The very first sentence of the executive summary states:

"Deterrence rests on the notion of ‘unacceptable loss’ – the ability to inflict a level of damage that a potential aggressor would judge outweighed any benefit they might gain by a particular course of action."

Well, yes and no. It does not just rest on the notion of unacceptable loss; it rests on the twin notions of unacceptable loss and unavoidable loss. That is where the whole concept of continuous-at-sea deterrence is central, because if one thinks one has a chance of avoiding an unacceptable level of retaliation, one might well take that chance in the hope that one will not have to face up to it.

I have quoted before, and I will quote it again tonight, what was stated the first time a senior British defence specialist considered the concept of what in those days would have been called atomic deterrence. That was in June 1945 in a top secret report drawn up by a committee of defence scientists headed by Professor Sir Henry Tizard. He made a comparison between the atomic bomb, which at the time had not yet been tested or used against Japan, and the concept and practice of duelling:

"Duelling was a recognised method of settling quarrels between men of high social standing so long as the duellists stood twenty paces apart and fired at each other with pistols of a primitive type. If the rule had been that they should stand a yard apart with pistols at each other’s hearts, we doubt whether it would long have remained a recognised method of settling affairs of honour."

However, if the duellists do not know whether the pistol is loaded, then even if they are standing only a yard apart they might just be reckless enough – "reckless" is the word that we hear time and again in the context of this Lib Dem policy – to take a chance. The whole point about nuclear deterrence is that it is unacceptable and unavoidable that a country will suffer nuclear destruction if it uses its nuclear weapons against a similarly armed country.

In the document, which was prepared by two civil servants in the Cabinet Office specially seconded from the Ministry of Defence, a number of strange concepts are articulated. One of them is familiar – continuous deterrence, which is referred to without quotation marks. Then the document refers to things called "focused deterrence", "sustained deterrence", "responsive deterrence" and "preserved deterrence". I have studied this subject for at least 31 years and I have never come across those terms before. At a briefing earlier today, the two civil servants were good enough to admit that in fact they had made them up. That is perfectly okay, except for one thing – the use of the word "deterrence". They could just as easily have referred to something like "intermittent deterrence", "semi-deterrence", "microscopic deterrence" or "virtually zero deterrence". It is not really deterrence unless it is certain; that is why it used to be called "mutually assured destruction". It is not enough to be able to threaten destruction; it has to be assured because otherwise the person may not be deterred.

It may seem as though the Liberals’ policy is in disarray, but they could still emerge, at the end of this process, as the winners. I will explain why. At the next general election, we could have another hung Parliament, as my hon. Friend the Member for Plymouth, Sutton and Devonport (Oliver Colvile) suggested. The Liberal Democrats could then say to the Leader of the Opposition, "All that stands between you and entering No. 10 Downing street is to get rid of this weapons system." They would not say, "Go down to two boats"; they would say, "Get rid of it completely", because that is what they have wanted all along.

Hugh Bayley: In the unlikely scenario that the hon. Gentleman paints of our having another hung Parliament, the Liberal Democrats would presumably negotiate both with his party and with mine. I think he is going to give me a firm view of what the answer would be from his party, and our Front Benchers have already given a firm view of what the answer would be from our party.

Dr Lewis: I am delighted by that intervention, because it not only gives me an extra minute but anticipates the next part of my argument.

If the Leader of the Opposition accepted that deal, then knowing the Liberal Democrats, they would start making the same offer to the current Prime Minister, who would have to think to himself, "Well, if I say no and the leader of the Labour Party has said yes, Trident is doomed anyway, so I may as well say yes as well." Who knows how these things might work out?

However, a solution is at hand: we could sign the main-gate contracts for some or all of the submarines in advance of the next general election. The only reason we put that off was to enable the Liberal Democrats to have their alternative study. They have had their alternative study, and it did not even consider a two-boat solution; it considered only a three-boat or four-boat solution. It could hardly be a breach of the coalition agreement if we were to challenge the Liberal Democrats to accept signing the contracts on the first two boats, if not the first three. That would at least prevent them from blackmailing either party, in the event of a hung Parliament, to get rid of the deterrent entirely.

At the most recent Defence questions I think I heard from the Opposition a commitment to try to bring forward the main gate decision to this side of the election. I urge Opposition Members who believe in deterrence to join Conservative Members and put relentless pressure on our leaders for a grand coalition to bring forward the main gate decision and secure the future of the nuclear deterrent.