When the policy was launched soon after the general election Mr Cook was heavily criticised by campaigning groups for failing to be tough enough on the arms trade.
Now the book - not authorised but written with the co- operation of the Foreign Secre- tary and his staff - has suggested Mr Blair was behind the Government's more arms-friendly stance. One of his main advisers was Lord Hollick, a Labour peer and former director of British Aerospace.
John Kampfner, the biography's author, suggests that while Mr Cook's junior minister, Tony Lloyd, was being lobbied by anti-arms pressure groups, Mr Blair was entertaining the heads of major arms companies in Downing Street.
"These captains of industry were the kind of people Blair liked to invite around to Downing Street," the book says, claiming a final draft of Mr Cook's ethical arms policy was toned down by officials in Downing Street, then by Mr Blair. In a meeting with his Foreign Secretary, the Prime Minister insisted on the insertion of a statement underlining the government's commitment to a strong defence industry.
Downing Street also added a rider that the use of defence equipment by security forces for their own protection was not to be considered grounds for refusing an export licence. Mr Cook is believed to have protested to little effect at the dilution of his reforms, for which he campaigned for two decades in opposition.
Ann Clwyd, the Labour MP for Cynon Valley who has pressed for curbs on the arms trade, said the book confirmed that Robin Cook had acted in good faith. "People were whispering to me that I didn't understand the kind of pressures Robin was under," she said. "Presumably, that pressure came from Number 10."
A spokesman for British Aerospace said its senior executives met government regularly, but policy was a matter for ministers. Lord Hollick had resigned his directorship for political reasons.
Downing Street declined to comment.