The evolutionary biologist Prof Richard Dawkins admitted he was 'disappointed' that he had not topped the bidding for a letter by Albert Einstein in which the famous physicist branded religious beliefs as "childish superstitions", it emerged today.

The letter fetched a record £170,000 at auction yesterday – four times higher than the previous price for an Einstein letter – after being bought by an overseas private collector.

Bloomsbury Auctions in Mayfair, which handled the sale, was deluged with interest in the lot, which had been given a guide price of between £6,000 and £8,000.

"What surprises me is the extraordinarily low estimate the auction house originally gave," Dawkins told the Guardian. He said he was disappointed to have lost the bid but added, "In a way I'm delighted that such a thing should be so highly valued."

Dawkins said he had planned to purchase the letter with his own money and present it to the Richard Dawkins Foundation for Reason and Science.

"This letter was about something very important to Einstein I suspect."

Dawkins was cagey about the precise value of his bid saying only that, "it was substantially higher than the estimate but substantially lower than the final price."

Unable to attend the auction, he put in his bid remotely. "I was closeted all afternoon unable to hear any news. I didn't get any news of what had happened until I logged onto the Guardian website just after midnight and saw the news item there."

Rupert Powell, managing director of Bloomsbury Auctions, said the lot had received unprecedented interest from around the world. "[The sale price] beats the world record for an Einstein letter by about 4 times," he said. "It's a massive difference."

The auction house installed an extra 11 phone lines in order to include international bidders. Powell said the atmosphere in the saleroom went from excitement, to disappointment – as various bidders dropped out – to disbelief at the rocketing price.

The £170,000 value is merely the so-called hammer price; with auction house fees and other costs the buyer paid a total of £207,600.

The letter clearly illustrates Einstein's views on the supernatural. "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. No interpretation no matter how subtle can (for me) change this."

Einstein, who was Jewish and had declined an offer to be Israel's second president, also rejected the idea that the Jews are God's favoured people.

"For me the Jewish religion, like all others, is an incarnation of the most childish superstitions. And the Jewish people to whom I gladly belong and with whose mentality I have a deep affinity have no different quality for me than all other people. As far as my experience goes, they are no better than other human groups, although they are protected from the worst cancers by a lack of power. Otherwise I cannot see anything 'chosen' about them."

Powell said he thought the letter had captured buyers' imagination because it is such a clear exposition of the great physicist's views.

"One of the greatest scientific/philosophical minds of all time is succinctly putting his belief in fundamental questions," he said. "Those questions about God and religion and Judaism are as relevant today as they were 50 years ago when he wrote it."

The £170,000 price tag is exceptional. In 2007, a set of 13 letters and three holograph post cards was sold for $60,000 (£30,000) and an Einstein letter on world government went for $9,000; and in 2006 a six-page scientific essay plus a letter fetched £300,000.