SECRECY is part of Britain’s nuclear deterrent, Sir Michael Fallon, has insisted as he made clear that there were certain things the public had to trust the UK Government on and Trident “is one of them”.

In an interview with The Herald, the Defence Secretary also declined to comment in detail on what the cost was to the taxpayer of the fall in sterling in terms of buying military equipment, saying that any suggestion it could be £1 billion a year was “speculation”. He insisted the public should be reassured that the Ministry of Defence took “precautions” against currency fluctuations but declined to elaborate.

Last month, Perth-born Sir Michael was forced to the Commons dispatch box to respond to MPs’ questions following a report that in June an unarmed Trident missile had misfired during a training exercise off the coast of America.

Read more: Theresa May rules out Scots deal on immigration despite warnings

The MoD insisted that HMS Vengeance and its crew had been “successfully tested” but, when MPs asked about the alleged misfiring, the Secretary of State steadfastly refused to give any “operational details of the demonstration and shakedown operation”.

In an interview with The Herald Sir Michael was asked if the public had a right to know that Britain’s nuclear deterrent worked or not, to which he replied: “I have made it clear that we are in no doubt about the effectiveness of the deterrent. So, the answer to that is Yes. The public has every right to be reassured that we have a deterrent that’s capable and effective.”

But he was asked how could the public, through its elected representatives, hold the Government to account on Trident when secrecy was at the heart of its policy on the nuclear deterrent.

Read more: Theresa May rules out Scots deal on immigration despite warnings

Sir Michael declared: “Secrecy is part of the deterrent. The deterrent’s credibility obviously depends on us keeping its operation as secure as possible, so there is obviously a compromise there to keep it secret.”

When it was suggested that the Government’s approach was simply to tell the public: ‘Just trust us,’ the Defence Secretary said: “There are some things that people have to trust the Government on and the nuclear deterrent is one of them.

“But, let me just reassure you, that we would not have asked Parliament to renew the submarine fleet if we had had any doubts about the safety, capability or effectiveness of the deterrent.”

Asked if the reporting of the alleged misfiring had been irresponsible, Sir Michael replied: “Any discussion of the details of the deterrent can only help our adversaries.”

Asked if the reports had undermined the effectiveness of the deterrent, he said: “I have made it clear that we’re not discussing in public details of the deterrent or the way it operates or details of how the different systems were tested. All the rest is speculation but that speculation is not helpful.”

Read more: Theresa May rules out Scots deal on immigration despite warnings

The Cabinet Minister was also asked about the impact on defence procurement of the dramatic fall in sterling following last summer’s Brexit vote since when the pound against the dollar has dropped by around 20 per cent.

“Like any other large organisation we take precautions against fluctuations in the currency and it’s too early to be certain as to exactly where the pound is likely to settle against either the dollar or the euro. It’s not something I’m going to comment on I’m afraid,” said Sir Michael.

In August, RUSI, the respected defence think-tank, estimated that the defence costs to the Treasury because of the fall in sterling would rise by around £700 million a year; yet since then the pound has fallen even further.

Asked if the additional cost could now be topping £1bn a year, Sir Michael said: “That is speculation and speculation about future movements in the currency. I can reassure you, we take precautions against fluctuations in the currency like any large organisation.”

But when it was suggested a fluctuation of 20 per cent was quite a significant move, he noted: “I can’t add to what I’ve said.”