A letter from Albert Einstein, which encouraged the US to develop the world's first nuclear bombs, is to go up for auction.

Written to President Franklin D Roosevelt, in 1939, the note warns that Nazi Germany might be able to create such weapons - and suggests the US begins its own atomic programme.

Three years later, America began the Manhattan Project, which led to the first ever use of atomic weapons, against Japan, in 1945.

The letter is being sold as part of an auction of artefacts belonging to Microsoft co-founder Paul Allen, who died in 2018 at the age 65.

Bill Gates, who started the company with Mr Allen, said: "Personal computing would not have existed without him."

His personal collection is go on sale at Christie's, in New York, in September.

There will be a range of items reflecting his interest in - and influence on - computing, but the Einstein letter is expected to be the centrepiece.

It has been given an estimated value of between $4m and $6m (£3.2m-£4.7m).

It was actually written by Hungarian physicist Leo Szilard with help from other scientists - but it was signed by Einstein because his status as one of the greatest scientific figures of all time made it more likely to get the president's attention.

| Getty Images: Einstein signed the letter, written by physicist Leo Szilard (right)

Dated 2 August 1939 - just weeks before the Second World War was declared - it warns that Germany may have worked out the science to create a nuclear bomb.

"It is conceivable - though much less certain - that extremely powerful bombs of a new type may thus be constructed," it says.

But Einstein is reported to have later regretted the letter due to its role in making America the only country - at the time - to make nuclear weapons.

He was quoted in 1947 as saying: "Had I known that the Germans would not succeed in producing an atomic bomb, I would never have lifted a finger."

| Getty Images: A PDP-10 computer similar to that which is going on sale

Also being auctioned is a computer from 1971, a DEC PDP-10: KI-10 model which Mr Allen helped restore.

Christie's says this is similar to a model which he would have worked on with Bill Gates when they founded Microsoft.

These types of computers were key in the development in early forms of the internet.

The computer is expected to fetch between $30,000 and $50,000.

And a spacesuit belonging to astronaut Ed White - who became the first American to perform a spacewalk, in 1965 - has an estimated value of $80,000 to $120,000.

| Getty Images: In 1965, Ed White became the first American to perform a spacewalk

In 2022, Mr Allen's art collection set a new world record for the biggest sale in an auction, raising over $1.5bn (£1.2bn).

It included works by Pablo Picasso, Salvador Dali and Vincent Van Gogh.

It was his wish that the proceeds would go to philanthropic causes.

During his life, he donated more than $2bn to charitable causes, set up art and culture museums and also owned various American sports teams.

His net worth at the time of his death was estimated at over $20bn by Forbes.

| Getty Images: Paul Allen (left) with Bill Gates in 1984