Dominic Raab has been urged to block the appointment of a Rwandan diplomat in London for his alleged role in detaining a government critic.
Paul Rusesabagina, who was portrayed in the Hollywood film Hotel Rwanda, used his Kigali hotel to save hundreds of ethnic Tutsis during the 1994 genocide.
Last year he was tricked into flying to Rwanda to face terrorism offences.
Rwanda's proposed new high commissioner has insisted his role in getting Mr Rusesabagina to the country was legal.
Johnston Busingye was, until recently, Rwanda's justice minister.
A spokesman for the Rwandan government said Mr Busingye had "served with distinction" in his former roles" and added that Mr Rusesabagina's arrest for terrorism offences "complied with all domestic and international law".
In August 2020, Mr Rusesabagina - who had been living in exile in the United States - got on a private jet in Dubai thinking it was heading for Burundi.
Instead the flight actually landed in Rwanda - something his supporters have described as a kidnapping.
In an interview with Al Jazeera in February 2021, Mr Busingye said "the government paid" for the flight taking Mr Rusesabagina to Kigali and paid for the individual who tricked him onto the plane:
"The payment was to facilitate the transportation of that man… to facilitate the plan of this man to transport Rusesabagina to Rwanda," he said.
Asked if this was legal, Mr Busingye replied: "Yes."
After spending eight years at the Justice ministry, Mr Busingye was abruptly removed from his post earlier this month by Rwandan President Paul Kagame, only weeks before a court in Rwanda is expected to rule on the charges against Mr Rusesabagina.
Mr Rusesabagina, a Belgian citizen now but with US residency, has been a vocal critic of Rwanda's authoritarian president.
The 66-year-old former hotel manager, who is reportedly in ill health, faces various charges of terrorism and founding an armed group responsible for terror attacks in Rwanda.
Prosecutors have asked for a life sentence, but Mr Rusesabagina supporters dismiss the proceedings as a "sham trial".
The Lantos Foundation, a prominent human rights organisation in the US, has already urged the US authorities to impose so-called Magnitsky sanctions against Mr Busingye for the "significant role" they claim he played in "grave human rights violations".
But now the foundation - set up by the late Tom Lantos, the only Holocaust survivor to become a US Congressman - has also written to Foreign Secretary Dominic Raab urging him to reject the credentials presented by Mr Busingye and impose UK sanctions on the former minister.
Dr Katrina Lantos Swett, president of the foundation, said: "President Paul Kagame may believe that by sending Johnston Busingye to London, he can divert the focus from the former justice minister's shameful actions and the way he brazenly violated the basic human rights of a humanitarian hero.
"But nations who respect human rights cannot allow Kagame to try and sweep these violations under the rug by simply assigning Mr Busingye elsewhere."
She said the foundation called on Mr Raab to refuse to accept Mr Busingye as Rwandan ambassador.
"Furthermore, the British government should seriously and thoroughly investigate the circumstances of Mr. Rusesabagina's kidnapping and Mr Busingye's involvement in these events," she added.
Under the Vienna Convention, ambassadors and high commissioners can take up their posts at the Court of St James only if the Foreign Office formally gives what is called its "agrément".
Only then can the appointment be approved by the Queen.
All applications are considered on a case-by-case basis and a number of factors are taken into account before a decision is made.
The Foreign Office refused to comment but it is understood that it has yet to grant its agrément to Mr Busingye.
Yolande Makolo, a spokesperson for the government of Rwanda, said: "Johnston Busingye, Rwanda's high commissioner-designate to the United Kingdom, has served with distinction as justice minister and attorney-general since 2013, and previously as a president/judge of the high court and principal judge of the East African Court of Justice.
"Paul Rusesabagina's arrest on an outstanding warrant for terrorism and related offences complied with all domestic and international laws, and he has since received a fair and transparent trial along with 20 co-accused of the FLN armed group which he led."