Experts from the Government's GCHQ spy centre are to deploy teams of "ethical hackers" in a bid to uncover holes in Downing Street's IT security.
The move follows the leak of a confidential memo from the prime minister.
Tony Blair and his officials fear the document - which outlined Blair's fears that Labour's message was not getting across - may have been obtained by hacking in to his IT system.
Two other confidential Downing Street memos have also entered the public domain, as have letters from other Cabinet members.
GCHQ's central security group has previously concentrated on military IT systems, but Blair has now decided to use its expertise across government.
Users in Downing Street and at the highest level of government will be given a special identification code which leaves a unique trace whenever they read, print, or call up a document or e-mail.
In addition, all departments commissioning a computer system will be issued with a "risk evaluation" outlining what security measures are needed.
Particular attention is being paid to the security of the Knowledge Network, the Whitehall database of up-to-date information for ministers and senior officials.