For a profession whose purpose is to find out information, spies know surprisingly little about their own past. But secret agents now have no excuse not to know about their antecedents.

Professor Christopher Andrew, the official historian of MI5, has charted the evolution of espionage from the Old Testament in a new book.

“There was no profession that knew so little about its own history as the intelligence community did,” he says.

The role of Britain’s security services is evolving, with GCHQ coming out of the shadows to defend us against cyber-hackers attempting to shut down our power stations or disrupt our markets.

And were a Labour government elected, the relationship between Prime Minister and Britain’s spy agencies could become very different.