Prince Nasser of Bahrain is not immune from prosecution over torture claims, the High Court in London has ruled.
Prince Nasser bin Hamad al-Khalifa has been accused of being involved in the torture of prisoners during a pro-democracy uprising in Bahrain in 2011.
Judges overturned the Crown Prosecution Service's decision that the prince had state immunity from prosecution.
The Bahrain government said it "categorically denies" the claims, calling them politically motivated.
The Crown Prosecution Service (CPS) said the issues raised by the judicial review were "academic" because the police would have to launch an investigation first, and the possibility of immunity was not one of the reasons for them not doing so.
A dossier of torture allegations, dating back to 2011, had been given to the CPS in 2012 while the prince was in the UK for the London Olympics.
The arrest and prosecution of the prince was then sought. However, he was allowed to return to Bahrain after the CPS decided he had diplomatic immunity.
Clive Coleman, BBC legal affairs correspondent
Under the UN Convention Against Torture and other Cruel, Inhuman and Degrading Treatment and Punishment 1987, states must criminalise torture and pursue public officials of other nations when they are present in the state's territory.
In other words, states are justified in prosecuting torture wherever it takes place because the offenders are - as was said in one well-known case - "common enemies of all mankind and all nations have an equal interest in their apprehension and prosecution".
A principle was established in the case involving former Chilean dictator, General Augusto Pinochet, that there be "no safe haven" for former public officials involved in torture or crimes against humanity.
There are very limited exceptions for heads of state, heads of diplomatic missions and their families, and foreign ministers.
The case arose after a refugee from Bahrain, referred to as FF, sought the arrest of Prince Nasser.
FF said he had been tortured by the Bahraini authorities - but not by Prince Nasser directly - during a pro-democracy uprising by the majority Shia community between February and March 2011.
Protesters had demanded more rights and an end to claimed discrimination against the community by the Sunni royal family.
The anti-government demonstrations led to a crackdown by the Bahraini authorities and the deaths of several protesters.
In a statement, FF said the prince had now "lost his immunity" and will need to "consider the risk of investigation, arrest and prosecution when he is travelling outside Bahrain".
FF's lawyers said the police could now conduct an investigation the next time the prince - a "frequent visitor" to the UK - entered the country.
A Bahrain government spokeswoman said: "The Crown Prosecution Service said the decision on immunity was academic as it had solid fact-related grounds for the basis on which it determined it could not prosecute Sheikh Nasser.
"All this was made plain in court today. In short, the situation has not, and will not, change as there is no evidence for the allegations."
Deborah Walsh, deputy head of special crime and counter-terrorism at the CPS, said it could "no longer maintain our position that the prince could have immunity, in line with recent case law on this issue".
She said: "We have always maintained that the issues raised by this judicial review are academic as before the DPP can consent to any application for a private arrest warrant, there needs to be an investigation by police.
"The likelihood of immunity is not considered a bar to prosecution and is a matter that should be considered on a case's individual facts and merits, after some investigation."