The two lawsuits - filed at Britain's High Court - accuse the attorney general, the MI5 security service and MI6 secret intelligence service of being complicit in the abduction, treatment and interrogation of the eight men, according to The Daily Mail newspaper.
All eight were detained in Afghanistan, Pakistan or Gambia at various times and were transferred for detention at the U.S. military prison at Guantanamo Bay Naval Base, on the southeast tip of Cuba.
They claim in the lawsuits that British authorities knew they would be taken to Guantanamo, but nevertheless cooperated with the Americans, the newspaper said.
"It is culpability by the British authorities in being involved in most of the process, their presence on every step of the journey before we got to Guantanamo," one of the former detainees involved in the suit, Moazzam Begg, was quoted as saying.
Begg told The Associated Press on Saturday he had been advised by lawyers not to comment further on the case.
Birnberg Pierce, the law firm named as acting for the men, refused to confirm the report when contacted by the AP. The High Court was closed, and Britain's Home Office said it could not comment on the case, as no writs had yet been served on the government.
The group has filed two separate writs against the government, MI5 and MI6 - one on behalf of five Britons, and the other on behalf of three foreign citizens with British residency, the newspaper said.
One names five Britons as claimants: Moazzam Begg, released in 2005; Richard Belmar, and three youths known as the "Tipton Three" after the town in England where they are from - Ruhal Ahmed, Shafiq Rasul and Asif Iqbal.
The three youths' story was told in "The Road to Guantanamo," a film by British director Michael Winterbottom. They tried unsuccessfully to sue the U.S. government for $10 million (6.3 million euros) each in damages.
The other writ was filed on behalf of Libyan Omar Deghayes and Jordanian Jamil el-Banna - both released in December - and Iraqi Bisher al-Rawi, released last year.
Lawyers for el-Banna and al-Rawi have previously claimed that U.K. officials instigated arrests that led to the two being detained and transferred to Guantanamo.
Al-Rawi and el-Banna were arrested in Gambia in 2002 while trying to return to Britain with electronic equipment. Their lawyers say British officials mistook a battery charger they carried for part of an improvised bomb when they first left the U.K. and passed those concerns on to overseas intelligence agents.