LONDON (REUTERS) - Britain urged an independent investigation into the violence against protesters in Hong Kong opposed to a Bill that would allow people to be sent to mainland China for trial, warning on Tuesday (June 25) that the fate of its former colony would be a litmus test for China.

Millions of people have clogged the streets of the Asian financial centre this month to rally against the Bill, which would allow people to be extradited to the mainland to face trial in courts controlled by the Communist Party.

Hong Kong police fired rubber bullets and tear gas at demonstrators who threw plastic bottles.

"I today urge the Hong Kong... government to establish a robust, independent investigation into the violent scenes that we saw," Foreign Secretary Jeremy Hunt said in Parliament.

"The outcome of that investigation will inform our assessment of future export licence applications to the Hong Kong police and we will not issue any further export licences for crowd control equipment to Hong Kong unless we are satisfied that concerns raised on human rights and fundamental freedoms have been thoroughly addressed."

Hong Kong returned to Chinese rule in 1997, since when it has been governed under a "one country, two systems" formula that allows it freedoms not enjoyed on the mainland, including freedom to protest and a much-cherished independent judiciary.

"We will stand by that agreement and we expect China to do the same," Mr Hunt said. "What happens in Hong Kong is, I think for all of us, a litmus test of the direction of travel that China goes in."

"The fundamental freedoms of Hong Kong are what have made it such a stunning success since 1997 and indeed before 1997 - anything that contradicted the letter or spirit of the basic law that preserves them should not happen," Mr Hunt added.