Image copyright AFP Image caption Pro-democracy groups celebrated after hearing Leung Chun-ying would not stand again

Pro-democracy groups in Hong Kong have made record gains on the Election Committee, which will decide the territory's new leader.

Amid a high turnout they have won more than 300 seats - about a quarter - reports say, although pro-Beijing groups will still have a big majority.

Critics say the results show the undemocratic nature of the system.

Hong Kong's Financial Secretary John Tsang has resigned his post, reportedly to run for the leadership in March.

Mr Tsang is regarded as a more moderate alternative to current leader Leung Chun-ying, who has said he will not seek re-election.

Image copyright AFP Image caption John Tsang (right) is expected to announce his bid to succeed Mr Leung (left)

Known as "Uncle Pringles" for a moustache similar to one worn by a character on a potato crisp brand's packaging, Mr Tsang is a US-educated fencing and martial arts enthusiast who has been the finance minister since July 2007.

Candidates running for chief executive have to be nominated by the 1,200-strong Election Committee - and need a majority of votes from the committee to win.

The committee's members are made up of delegates from special interest groups.

Analysts say the pro-Beijing majority ensures the battle will be between figures favourable to the mainland government. Although none have yet declared, Mr Tsang, former security secretary Regina Ip and chief secretary Carrie Lam could face off.

The BBC's Helier Cheung, in Hong Kong, says pan-democrats will argue that their strong performance is a reflection of dissatisfaction with the current government, and the slow pace of democratic reform.

In 2014, tens of thousands of people demonstrated for weeks to demand a one-person, one-vote election for chief executive, but they failed to win any concessions from Beijing.