Staff will wear the pants in Lush stores all of this week in the hope of highlighting the illegal detention and torture of men held in the prison.
Mark Constantine, Lush co-founder, decided to launch the campaign after learning about two current detainees -- 'Al Jazeera News' cameraman Sami Al Haj and British resident Binyam Mohamed, who have been held without trial for five years.
Constantine learned of the two men when he met Clive Stafford Smith, human rights lawyer and director of legal aid charity Reprieve, who is representing Al Haj and Mohamed in Guantanamo.
Stafford Smith said the US government had absurdly accused him of sneaking in contraband underpants to his clients; this gave Reprieve the idea to print the slogan "Fair Trial my Arse" on a pair of briefs.
He said: "The most effective counter-terrorism strategy any government can employ is the universal enforcement of human rights and Reprieve is delighted that the staff at Lush have taken up the fight for justice for the prisoners in Guantanamo Bay."
Lush has also created a new fizzing bath ball called 'Guantanamo Garden', which has been invented to tell the story of the Guantanamo prisoners who have been taking seeds from their meals and planting them in the outdoor holding cell to create their own garden of watermelon, peppers and a lemon tree.
The ball is coloured orange to represent the jump suits worn by the prisoners and when it is dropped into the bath, a hidden photo of Al Haj and Mohamed floats to the top bearing copy urging customers to take action by logging onto Reprieve's .
Constantine said: "Because Lush is a cosmetics company we normally campaign over the rights of mice and rabbits being harmed in pointless and cruel safety experiments.
"But when humans are being treated worse than rats in a cage we knew it was time to launch an initiative to close Guantanamo."
Lush will also be donating 100% of the proceeds of its Charity Pot hand and body cream sold this week to Reprieve.