Martin Mubanga, 32, said in a newspaper interview on Sunday that during one interrogation session, he was forced to relieve himself in the corner of a room, after which his interrogators soaked up the urine with a mop and daubed him methodically.
"All the while he was racially abusing me, cussing me, saying: 'Oh, the poor little nigger'," Mubanga,born in Zambia but raised in Britain, told The Observer.
"He seemed to think it was funny."
Mubanga said he was forced to endure a "hot room" where the temperature was turned up to 37.7 degrees centigrade, leaving him severely dehydrated.
Mubanga and three other Britons were returned to Britain on 26 January after being held at the Guantanamo Bay detention centre for almost three years. While the others were arrested in Afghanistan and Pakistan, Mubanga was detained in Zambia, where he had gone to study Islam.
He said that after his arrest by Zambian security officials, he was questioned by a female American official and by a British man who claimed to be working for MI6, Britain's foreign intelligence agency.
Mubanga's lawyer, Louise Christian, said she intends to take legal action against the British government, citing violations of British, Zambian and international law.
"We are hoping to issue proceedings for the misfeasance of officials who colluded with the Americans in effectively kidnapping him and taking him to Guantanamo," she said.