The comments on Tuesday come after London arrested and pledged to deport 10 foreigners in a crackdown on hardline Muslims after the deadly 7 July bombings in the city.
To facilitate the process, Prime Minister Tony Blair's government signed a non-legally binding agreement with Jordan that allows the two countries to deport undesirables from one to the other without fear of torture.
Blair plans to strike similar accords with nine other countries, including Egypt and Algeria.
"The United Kingdom cannot deport security suspects to Jordan without violating the international prohibition against sending persons to countries where they face a serious risk of torture," HRW said.
The US-based rights group rubbished the 10 August memorandum of understanding between Britain and Jordan as meaningless.
"There is still torture in Jordan, especially with regard to security suspects," Joe Stork, deputy director of HRW's Middle East division, said in a statement.
Home Secretary Charles Clarke (L)
says the UK remains worried
"All the good reasons that prevented the UK from deporting people to Jordan before 10 August remain unchanged by this agreement," he said.
Britain and Jordan are both signatories to the Convention Against Torture and Cruel, Inhuman or Degrading Treatment.
The text also forbids the transfer, return or expulsion of individuals to countries where they run a substantial risk of being tortured.
HRW accused London and Amman of using their accord in a bid to skirt their international legal obligations.
"Jordan stands to gain custody of criminal suspects while Britain rids itself of unwelcome persons, and neither country has any incentive to monitor treatment or investigate allegations of abuse," it said.
Amman has already said it plans to charge one of the 10 detainees - Jordanian-born Abu Qatada, 44 - with plotting to stage terrorist attacks when he is extradited from Britain.
HRW, however, said court documents from March 2004 showed that the British Home Office "recognised ... that he (Abu Qatada) cannot be returned to Jordan since he would be likely to face persecution or breaches of his human rights under Article 3 of the European Convention on Human Rights."
The human rights watchdog listed cases of alleged torture in Jordanian prisons to highlight its fears.
It said that in September last year, an official body called the National Human Rights Centre announced that Abdullah al-Mushaqaba had died in Juwaida prison as a result of torture.
The centre also received 250 allegations of torture or ill-treatment in Jordanian jails, according to HRW, which noted that the figure did not include the General Intelligence Directorate, where security prisoners are first taken.
Fifty-six people were killed in the
7 July London bombings
HRW said the diplomatic agreement between Britain and Jordan failed to offer sufficient safeguards against torture because such treatment is nearly always secretive and neither country has an incentive to blow the whistle.
In addition, the memorandum lacked effective tools to secure compliance, the group said, urging against similar accords with other countries in the region.
"Jordan, Egypt and Algeria all have a documented history of torture," said Stork. "Neither Britain nor any other country should consider returning people to such countries where they face the risk of torture."
Meanwhile, British Home Secretary Charles Clarke said on Monday that the UK remained "worried" by the prospect of a repeat of last month's deadly bombings in London despite the swift pace of a continuing police investigation.
"We remain worried... It would be ridiculous for us to assume that a further act could not take place," Clarke said after a briefing at Scotland Yard with Sir Ian Blair, head of London's Metropolitan Police.
Fifty-six people were killed, including four bombers, in the 7 July attack on three Underground railway trains and a double-decker bus in the British capital.
Those attacks were followed two weeks later by a failed attempt to repeat the blasts. Police are holding three key suspects in connection with that incident, with a fourth detained in Rome pending extradition.
Blair's government has said the attacks bear the "hallmarks" of Osama bin Laden's al-Qaida network, but rejects assertions that they were the result of Britain's military presence in Iraq.