Amnesty International and Justice, in separate reports, have dismissed any case for extending the limit.

Amnesty has published ten reasons why it thinks extending pre-charge detention is a bad idea while Justice says that terrorism investigations in the US have not prevented police from charging suspects within 48 hours of their arrest.

It is thought that the government is considering extending the limit from 28 days to 56 days.

Amnesty argues that any extension would undermine basic rights and would lead to deteriorating relations with the Muslim community.

Amnesty International UK Director Kate Allen, said: "While the case for extending pre-charge detention is full of holes and losing support week on week, the case against both in principle and practice is very strong.

Justice has released details of ten high-profile terrorism cases in the US between 2002 and 2007 which show that every suspect charged with a criminal offence was charged within 48 hours of being arrested. The complex investigations include plots to destroy Sears Tower and Brooklyn Bridge.

Eric Metcalfe, Justice's director of human rights policy and author of the report, said: "If the FBI can charge a terror suspect in 48 hours, why do UK police need more than 28 days?"

The Conservatives oppose any move to extend the limit, with Shadow Home Secretary, David Davis, saying of the Justice report: "This drives a coach and horses through government claims that we need an extension of the period of detention without charge beyond 28 days, when the Americans, time and again, are charging suspects within 48 hours."

Meanwhile, Justice Secretary Jack Straw said any proposals to extend the 28-day limit would be "very proportionate".