LONDON (AFP) - Leading Hong Kong pro-democracy activists on Wednesday urged former colonial power Britain not to turn a blind eye to "attacks" on freedoms that were enshrined in the 1997 handover to China.

Former Hong Kong number two official Anson Chan and campaigner Martin Lee said the semi-autonomous city risked becoming "ungovernable" just over two weeks after half a million people marched in Hong Kong demanding reforms.

The pair told the Foreign Affairs Committee of the House of Commons, or lower chamber of the British parliament, that London had been failing in its responsibility towards democracy in Hong Kong.

"It is vital that the British government does not turn a blind eye to current developments in Hong Kong, as it has been doing in its recent six-monthly reports," Chan told the committee members.

"Seventeen years after the return of sovereignty, this concept is under serious attack," she added.

The pair heavily criticised a British parliamentary report earlier this month in which then-British foreign secretary William Hague said the city's unique constitutional framework has worked well and that there was no "perfect model" for electoral reform.

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 by Britain in 1997 are being eroded.

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

While China has promised universal suffrage in 2017, China says candidates must be picked by a nominating committee - raising fears among democracy advocates that only pro-Beijing figures will be allowed.

Anson said that "unless Hong Kong gets universal suffrage... there is a real danger of Hong Kong becoming ungovernable."

China meanwhile accused Britain of interference over Hong Kong after the two activists met Deputy Prime Minister Nick Clegg on Tuesday.