Leading Hong Kong pro-democracy activists on Sunday urged Britain to defend freedoms in its former colony instead of its "shameful" pursuit of trade with China.

Former Hong Kong number two official Anson Chan and campaigner Martin Lee made the remarks following a visit to London, weeks after a huge July 1 march demanding democratic reforms in Hong Kong.

"The British government has an undoubted responsibility towards Hong Kong," Lee, former chairman of the city's Democratic Party, said in a press conference at the airport. "There is no reason for Britain not to care about Hong Kong," he said.

He accused London of failing to speak up for the city.

"It appears to me that the British policy on Hong Kong is summarised in three words: more China trade. This is shameful," he said.

Concerns are growing that the freedoms Hong Kong was guaranteed under the "one country, two systems" deal when the city was handed back to China in 1997 are being eroded.

In its latest edition, the Economist magazine accused London of putting its trade interests with China ahead of its duty to stand up for its former colony, citing multi-billion-dollar deals signed during a state visit by Chinese Premier Li Keqiang in June.

"Britain's outsize diplomatic clout derives from its values as a democracy and its ability as a permanent member of the Security Council to galvanise coalitions," the magazine said.

"Nobody needed to spell out what from now on would be the terms of the relationship (between China and Britain): deals would flow, but only for as long as Britain kept its nose out of Chinese affairs."

It urged Britain to support Hong Kong's liberties and set an example for other countries to do likewise. "If Britain kow-tows to China, why should they bother?"

During their visit, the activists testified at a parliament foreign affairs committee. Their meeting with British Deputy Prime Minister Nick Clegg was criticised by Beijing as interfering with the city's affairs.

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Chan said it was a "disappointment" not being able to meet Prime Minister David Cameron.

"Britain has moral and legal responsibility towards Hong Kong," she said.

Fears over freedoms being eroded heightened in June when Beijing published a controversial "white paper" on Hong Kong's future, widely seen as a warning to the city not to overstep its bounds.

While China has promised universal suffrage in 2017 to choose the city's leader, it says candidates must be picked by a nominating committee -- raising fears among democracy advocates that only pro-Beijing figures will be allowed to stand.

Hong Kong's chief executive is currently chosen by a pro-Beijing committee.