The book has been encrypted, but Mr Tomlinson could release the cipher "keys" for worldwide reading in moments, by sending an e-mail from a phone booth anywhere.
Mr Tomlinson had been planning the release of 117 names of former and serving MI6 officers on to the Internet for weeks, although on Wednesday he denied posting the list.
But ministers have little doubt he was behind one of Britain's biggest security breaches for decades.
Last month Mr Tomlinson sent an e-mail to The Independent, reading: "I have decided that since I cannot take MI6 to court for blocking my entry to France (and Australia and USA) I have no option but to adopt a hardline response."
A week ago he left the Geneva hotel where he has been staying since last summer.
The book contains details of plans by MI6 to assassinate President Slobodan Milosevic in the early 1990s, and a British-run "mole" in a senior position in the German Bundesbank who leaked crucial information to Britain in economic negotiations.
Mr Tomlinson also claims that MI6 has a unit specifically for stealing military and economic secrets from European allies. These allegations have already been leaked, but Mr Tomlinson claims the book also contains new material that is potentially more embarrassing for the Government.
This new threat to expose the innermost secrets of Britain's spymasters follows desperate Whitehall attempts over the past week to prevent the list appearing on the Internet.
Mr Tomlinson, a 37-year-old Cambridge graduate who worked for MI6 between 1991 and 1995, was sacked then imprisoned in 1997 for breaking the Official Secrets Act.
The full list, including names of the head of MI6, a Cambridge don and an ex-minister's son, appeared yesterday in Internet discussion groups.
A spokesman for the Foreign Office admitted that the list's rapid spread around the Internet was now "a fact of life" and any efforts now would be "a damage limitation exercise". The spokesman called Mr Tomlinson "irresponsible and unbalanced", adding: "We do not negotiate with people who hold us to ransom."
But the Government claims that the list's publication "puts lives at risk" was challenged yesterday by the editor of Lobster magazine, an intelligence- watching journal.
Stephen Dorril said: "About 30 of the names were published in our magazine in 1989. It's absolute rubbish to say lives are at risk. When it comes to officers operating under `light cover', where the names are published in the public Diplomatic Lists, no serving MI6 officer has been killed in the field since the 1940s."Reuse content