Image caption The particles are encased in a clear plastic sphere

Ever wondered what happened to the UK's piece of the moon? Four fragments which were given to the UK as a present?

Newly-released National Archives' files reveal that three prime ministers were unsure what to do with the official gift - which was given to the country by US President Richard Nixon.

Mr Nixon gave the then UK PM Harold Wilson the particles in 1970 - a year after man first landed on the moon.

They remain encased in a clear plastic sphere within No 10 Downing Street.

The fragments had been brought back by Apollo 11, the first manned moon mission, and were mounted on a display stand alongside a silk union jack which had also travelled to the moon and back with astronauts Neil Armstrong and Buzz Aldrin.

'Modern design'

The files show a note from Wilson, who decided the dust should stay at No 10 "in perpetuity", and another from his successor Edward Heath, who was asked if he wanted it to stay in Downing Street to which he replied "yes".

The pillared drawing room was suggested, in view of the stand's "fairly modern design", but Heath wrote: "I will have a look. I don't much like the idea of it being in one of the public rooms".

Image copyright AP Image caption Neil Armstrong and Buzz Aldrin landed on the moon in 1969

In the event it was sent off on a tour of regional museums before ending up back at No 10.

The truth, it seems, was that it did not look very attractive and it was not very exciting.

In July 1979 - two months after her election - Mrs Thatcher was told the dust had been "languishing in a cupboard", the files show.

She was asked if she thought it should go to the Geological Museum on semi-permanent loan.

Napoleon's toothbrush

"Not yet," she wrote, and as a result one official suggested it could go outside the Cabinet Room.

The moon dust issue raised its head again in 1984, when the it was mentioned to Mrs Thatcher during a visit to the Science Museum.

The files suggest officials seem to have thought the museum was asking to have it permanently.

Someone rang the museum's director - Dame Margaret Weston - to be told: thanks, but no thanks.

The museum already had plenty of other moon rocks. Dame Margaret likened it to another curiosity in the museum's collection - a toothbrush once used by Napoleon.

Last week the Downing Street press office confirmed the moon dust is still at No 10.

But did not say where.

In 2012 Nasa disclosed that many pieces of moon rock given away as gifts to other countries were now lost, stolen or missing or unaccounted for..