The Democratic Party on Saturday confirmed that its vice-chairman, Lee Wing-tat, has left the SAR.
Earlier, the pro-Beijing Wen Wei Po reported that Lee has been seen at the airport boarding a flight to London on Friday night.
The party's chair, Lo Kin-hei, said the veteran democrat and former lawmaker isn't in the city, but he plans to try to contact him.
He said Lee has not resigned as the party vice-chairman - a role he took up in June.
The 65-year-old politician and pro-democracy activist served as a lawmaker for multiple terms between 1991 and 2012. He replaced Albert Ho as the chairman of the Democratic Party from 2004 to 2006.
In 2019, he was found guilty of inciting others to commit public nuisance during the pro-democracy Occupy protests in 2014, and was given a suspended jail sentence.