WITHIN hours of Theresa May entering Number 10 on Wednesday she will take one of the greatest decisions of her career: what to do in the event of a devastating nuclear attack.

The Letters of Last Resort will be penned by the new prime minister when she takes office - instructing the four submarine commanders of Britain's Trident nuclear fleet on what to do should she be killed and the country destroyed.

But Mrs May's decision on whether the country should fire back in such an event will never be publicly known.

David Cameron's letters will be destroyed, unopened, when he leaves office and so his suggested course of action will remain a mystery.

The moment the newly appointed prime minister writes the letters is said to be profound.

Former chief of defence staff Lord Guthrie said that Tony Blair "went quiet" when he had to write the letters.

In a BBC Radio 4 documentary, Lord Guthrie said: "I think quite honestly, like most prime ministers, he hadn't given a huge amount of thought to what this really meant. And it is actually an awesome responsibility.

"It really comes home to you that he could, if the circumstances demanded it, create devastation on a huge scale."

Writing the letters is one of a number of protocols that Mrs May will likely complete today.

MPs will vote on replacing the ageing submarines carrying the Trident nuclear missiles on July 18.

READ MORE: MPs to vote on Trident renewal