The former lord chancellor has added his concerns to the body of opinion doubting the government's plans to extend the length of time terror suspects can be held without charge.
Lord Falconer said the government had failed to make the case for extending pre-charge detention and increasing the present 28-day limit would only be justifiable after a major incident.
His comments are revealed in an email to Keith Vaz, chair of the home affairs select committee which has been hearing evidence on the case for extending detention without charge.
The leaked emails, seen by the Guardian and verified by friends, show the former lord chancellor was critical of home secretary Jacqui Smith's call to increase the limit in case the 28-day period proved insufficient some time in the future.
He argues an extension could only be justified if a major incident had occurred "which so increases the threat that the safety of the nation required it".
An extension could not be made on the basis of a single complex case but "if the major incident made it impossible for the police and CPS to deal with the number of cases with which they are confronted within the 28-day period."
Lord Falconer also objected to the proposed parliamentary safeguard, where MPs would have to approve holding a terror suspect beyond 28 days. He said it would be difficult for parliament to debate cases without prejudicing an ongoing investigation.
He joins Lord Goldsmith, the former attorney general, and Ken MacDonald, director of public prosecutions, in expressing doubts about the need to extend pre-charge detention.
Reporting to the home affairs select committee this week, home secretary Jacqui Smith admitted six of the 71 respondents to a government consultation had been in favour of increasing the 28-day limit.