By Jeremy Lovell

LONDON, June 7 (Reuters) - British Prime Minister Gordon Brown has made a last ditch appeal for support from his party for proposals to extend pre-charge detention for terrorism suspects to 42 days.

Brown's poll ratings are at an all-time low after the loss of a formerly safe parliamentary seat, a drubbing in local elections and with the economy slowing sharply, and defeat in the security vote on Wednesday could further dent his authority.

"In the legislation currently before parliament we have done everything in our power to protect the civil liberties of the individual against any arbitrary treatment, because in Britain liberty is and remains at the centre of our constitutional settlement," Brown wrote in a letter to his lawmakers released by his Downing Street office on Saturday.

"The challenge has been to make sure that, through proper judicial and parliamentary oversight, we both keep the public free from the threat to our security, and secure the fundamental liberties of the citizen."

It is the latest in a series of efforts Brown and his party managers have made to head off a threatened rebellion by Labour parliamentarians who see the legislation as an unacceptable attack on civil liberties in the name of security.

The law currently allows police to hold suspects in a terrorist investigation for 28 days before having to charge or release them.

But since suicide bomb attacks on London's transport system in July 2005, which killed 52 travellers and injured several hundred more, police have been pushing for the 42-day period.

Brown said in the letter the extension was justified by the increase in number, scope and complexity of terrorist plots under investigation.

"In 1997, 19 mobile phones, one computer and seven computer disks were seized in terrorist investigations. In 2006, 1,620 mobile phones, 353 computers and 2,541 computer disks were seized," he wrote.

Investigations in 2004 into Dhiren Barot, the head of an al Qaeda plot to carry out bomb attacks in Britain, involved the seizure of 270 computers and 2,000 disks as well as inquiries in seven other countries, Brown noted.

Barot was jailed in November 2006 for a minimum of 30 years.

Brown, who took over from Tony Blair last June after a decade as his finance minister, wrote that British security services were investigating 2,000 terrorist suspects, 30 plots and 200 organised terrorist networks.

To try to win support from party members who fear that under Brown's leadership they will lose their seats in the next election that is due within two years, he has offered parliamentary and legal oversight of individual cases. (Editing by Alison Williams)

AlertNet news is provided by