A computer expert warned police about the activities of two of the 7 July London bombers in 2003, he says.
The pair made "anti-Western" material, Mr Gilbertson said
Martin Gilbertson told the Guardian he was concerned by material Mohammad Sidique Khan and Shehzad Tanweer produced at a West Yorkshire bookshop.
He also told the BBC he wrote to local police to express his concerns about the "anti-Western" material but that no action was taken.
A West Yorkshire police spokesman said it was impossible to track the letter.
Mr Gilbertson, who is a former Hell's Angel, said he came into contact with the two when he worked on computers at the Islamic bookshop in Beeston, Leeds, where both the men lived.
They may have trusted him because they perceived him to be anti-government, Mr Gilbertson told BBC's Newsnight programme.
"I worked for them for nearly four years but they were really, very violently against the British society," he added.
"They actually tried to convert me to Islam.
"I was tired of the daily grind of being rammed down my throat that I was, in their own words, a Satanist because I didn't believe in Allah."
Mr Gilbertson says his concerns over the material they were producing were so strong that, in October 2003, he went to his local police station to report his fears.
He then put his concerns down in writing on the advice of officers, he said.
Fifty-two people were killed in the 7 July bomb attacks
"In 2003 I sent them some disks that had been produced by the bookshop in Beeston, which was very anti-Western, anti-Iraq, anti-everything and very anti-Semitic.
"Also included in it was a list of names, which included both Tanweer and Khan."
He was "not the only person in Beeston concerned about them", he added.
But West Yorkshire Police say there is now no trace of the letter or the list of names.
It says it will be impossible now to track what happened to his letter or to say whether Mr Gilbertson's information was acted upon.
Shadow home secretary David Davis said Mr Gilbertson's claims reinforced the need for an independent inquiry into the 7 July attacks.
"It is vital that the public can be confident that every action is being taken to protect their safety and if there are any weaknesses in our intelligence operations that these are corrected immediately," he said.
Fifty-two people were killed and hundreds injured by four suicide bombers on the London Underground and a bus in Tavistock Square on 7 July last year.