Bloomsbury Auctions says Friday that the handwritten letter sold to an overseas collector after frenetic bidding late Thursday in London.

The sale price of $404,000, including the buyer's premium, was more than 25 times the pre-sale estimate.

Bloomsbury did not identify the buyer, but managing director Rupert Powell said it was someone with "a passion for theoretical physics and all that that entails."

The letter was written to philosopher Eric Gutkind in January 1954, a year before Einstein's death.

In it, the Einstein said that "the word God is for me nothing more than the expression and product of human weaknesses, the Bible a collection of honourable but still primitive legends which are nevertheless pretty childish."

"This extraordinary letter seemed to strike a chord, and it gave a deep personal insight one of the greatest minds of the 20th century," Powell said.

Einstein experts say the letter supports the argument that the physicist held complex, agnostic views on religion. He rejected organized faith but often spoke of a spiritual force at work in the universe.

Einstein's most famous legacy is the special theory of relativity, which makes the point that a large amount of energy could be released from a tiny amount of matter, as expressed in the equation EMC2 (energy equals mass times the speed of light squared).

The theory changed the face of physics, allowing scientists to make predictions about space and paving the way for nuclear power and the atomic bomb.