A British judge on Tuesday ruled Bahrain's Prince Nasser bin Hamad Al-Khalifa does not have legal immunity from prosecution following a review requested by a Bahraini torture survivor.
"It's a victory for the people of Bahrain," Sayed Al-Wadaei, director of advocacy at the Bahrain Institute for Rights and Democracy campaign group, told AFP after the hearing.
"The day of accountability has come for Bahrain," he said.
The survivor's identity was kept secret in the proceedings in the High Court in London and the person was referred to only as "FF" in order to prevent possible reprisals.
Activists want British police to investigate allegations that the prince, one of the sons of the current ruler of Bahrain, was involved in torturing political prisoners.
Shiite-led anti-government protests against the Al-Khalifa ruling family were crushed in 2011.
The decision has no immediate effect on the prince, who is not being prosecuted and considers Britain a home-from-home.
It could, however, lead to his arrest if the case is pursued and it goes against a previous 2012 decision by British prosecutors that the prince did enjoy immunity.
A lawyer for the survivor, Sue Willman, said she expected a meeting with the British police's war crimes unit "in the next few weeks" to discuss taking the investigation further.
Willman said that the 27-year-old prince travels to Britain "frequently", particularly to attend equestrian events.
Al-Wadaei said the prince considers Britain "a second home".
Embassy officials declined to comment on the ruling.
The International Federation for Human Rights hailed the ruling as "a major breakthrough" and criticised French authorities for failing to respond to a similar complaint filed in France in August when the prince visited Paris.
"The new decision opens the way for further investigations in the UK," the Paris-based federation said in a statement.
The prince, who is also head of the Bahrain Olympic Committee, had travelled to France to take part in the World Equestrian Games as a member of the Bahraini national team.
The Berlin-based European Centre for Constitutional and Human Rights was involved in the London case as an interested party in the proceedings.
The centre had raised the torture allegations with British Prime Minister David Cameron around the time of the 2012 Olympics held in London.
Willman also said that while the decision applies only to Britain, it would be "influential" for other EU states.
Tiny but strategically-placed Bahrain, home to the US Navy's Fifth Fleet, remains deeply divided three years after the 2011 protests.
It continues to witness sporadic demonstrations which often spiral into clashes with police.
Bahrain police last week detained leading rights activist Nabil Rajab over tweets deemed offensive.
Rajab acknowledged that he was responsible for the remarks posted on his Twitter account, and "legal measures have been taken to refer him to the general prosecution," a ministry statement said.
It said Rajab "insulted official bodies in his tweets".
Bahrain human rights activist Maryam al-Khawaja also faces trial for assaulting police officers, although she has been bailed following a plea from civil society groups from around the world.
King Hamad has set a November 22 date for Bahrain's first parliamentary elections since the uprising and the authorities have proposed relaunching a national dialogue.
The proposal has received a frosty reception from the opposition, which wants a constitutional monarchy.