The British government faces a legal challenge over allegations it was complicit in the torture of Bangladeshi MP Salauddin Chowdhury, who was arrested by the country's security forces earlier this month.

Lawyers acting for the 63-year-old's family claim the training provided by British forces to Bangladesh's Rapid Action Battalion [RAB], which arrested Chowdury, places the UK in breach of its obligations under international law.

RAB members have been held responsible for hundreds of extrajudicial killings since the unit was established in 2004. The unit itself admits to being responsible for more than 600 deaths, which it euphemistically attributes to "crossfire".

Details of British support for RAB were revealed in US embassy cables released by WikiLeaks and reported by the Guardian. They show that Britain has been providing training in "investigative interviewing techniques" and "rules of engagement".

According to Amnesty International, Chowdury was detained and tortured by RAB officers and Bangladeshi intelligence officials at an apartment in the Banani district of Dhaka on 16 December. His captors are alleged to have brought with them a number of torture implements and a doctor, who is alleged to have revived him three times after he lost consciousnes. The mistreatment is said to have included applying electrodes to his genitals, beating him, slitting his stomach with razors and twisting his toenails and fingernails with pliers.

According to Amnesty, Chowdury was initially arrested for questioning over an arson attack in which a person died, and was then reported to have been charged with offences allegedly committed during Bangladesh's 1971 war of independence. He is now being held in prison in Bangladesh.

Today his son, Fayyaz Chowdury, called on the British government to intervene.

"The UK government is up to its neck in this case having provided practical and financial support to RAB who are responsible for my father's ill-treatment. I am a British national and they must now intervene to secure my father's immediate release from prison and a binding undertaking from the Bangladesh government that he will not be subjected to any further torture and ill-treatment or a patently unfair trial."

The Foreign Office has defended the training offered to RAB as "fully in line with our laws and our values". A spokesman sought to suggest it was providing only "human rights training" for RAB, although RAB's head of training told the Guardian he was unaware of any human rights training since he was appointed last June.

Phil Shiner, of Public Interest Lawyers, which is bringing the legal challenge on behalf of Fayyaz Chowdury said: "The UK government owe the clearest of international obligations to my client, a British citizen, in circumstances where they are complicit in the torture of people like Mr Salauddin Chowdury. These obligations reflect international law principles that prohibit states from aiding and assisting other states in international crimes such as torture and extrajudicial executions. The UK government must use all means at their disposal to secure the immediate release of my client's father and to ensure that he cannot face a kangaroo court for war crimes that he did not commit."