A sighting of an elusive Gestapo spy with a taste for the London high-life sparked a frantic hunt during World War II, newly released files show.
Moerz was the only Nazi spy in the UK to evade capture in WWII
The agent, who used the name Wilhelm Moerz, is thought to be the only Nazi spy in Britain who evaded capture.
Moerz was reportedly spotted getting into a taxi in London on 25 May, 1940.
"He is undoubtedly one of the cleverest secret agents the Germans have at the present time," say the official files, released by the National Archives.
As a result of the reported London sighting, police spent months hanging around London nightclubs and hotels while MI5 followed-up sightings from northern Scotland to southern England.
But the German agent - suspected of involvement in the capture of two British spies in the Dutch town of Venlo in 1939 - eluded them.
He is described in the files as a "thorough scoundrel", an "affected and somewhat overdressed individual" and a "very dangerous double-crosser".
Moerz first came to the attention of British agents in Prague in the late 1930s. He was believed to have denounced opponents of the Nazi regime, leading to their arrest or death.
But it was a report that he had been sighted getting into a taxi in Regent Street, London, that set alarm bells ringing.
Described as well-dressed, aged about 35, "but looks younger ... slim, even on the thin side" with "very prominent, projecting upper teeth", a photograph shows him wearing a dark tie with light stripes.
Alerts were sent to ports, airports and police forces - including one message signed by an intelligence official, Adam Snuffbox.
A letter to London's police chief said Moerz's arrest would be "a matter of the greatest significance" as he had likely come to lead German espionage efforts in Britain.
Two hotel receptionists and a nightclub hostess recalled encounters with Moerz and he was suspected of being in Edinburgh, Newcastle and Leeds and even on a ship from the Orkney Islands off northern Scotland in the following weeks.
Two policemen scoured London for more than two months.
"In the course of their duties, they have had to visit many restaurants, cafes and nightclubs, with the result that we are now faced with a rather formidable bill of costs and, I am afraid, still without any Wilhelm Moerz!" says a note from intelligence officer Dick White, who later became MI5 chief.
Some officials came to believe the Regent Street sighting had been false, although a hotel register and witness statements suggested Moerz had been in Britain earlier in the year.
By 1941, MI5 had officially given up looking for him.
But the files offer an intriguing post-script - a letter in a German newspaper in 1955 suggested Werner Mikkelsen, believed to be Moerz's real name, was living in Frankfurt.