Tsang Yok-sing, leader of the Beijing-backed Democratic Alliance for the Betterment of Hong Kong, said he was reviewing his role in the cabinet and might walk out. This was because he feared that the anti-subversion bill was eroding his support base.
The defection of a second key ally could be a fatal blow for Tung, who is battling to defuse the biggest political crisis in Hong Kong's six-year existence as an autonomous territory of China. Without parties like that of Tsang’s, the ruling coalition will find it difficult to push through bills blessed by Beijing, like the one on anti-subversion.
"I will not rule out quitting the Executive Council," Tsang said. "We are looking at all possibilities.
"This (anti-subversion bill) issue is very divisive, contentious and some people are very strongly against it," he said. But "since we are in full support of it, those who are against it will not support us in the coming elections," Tsang added.
A former school principal, Tsang apologised last week to try placate an angry public after he said those who had joined the huge anti-government march on 1 July were being misled.
Tsang's party has been a staunch supporter of the bill, and still says it should be passed, but after the rally it has tried steadily to distance itself from Tung's administration.
On Sunday, the head of the pro-business Liberal Party, James Tien, quit Tung's cabinet, forcing the chief executive into putting off the unpopular security bill and fuelling demands that Tung should resign as he was no longer able to govern.
Half a million people took to the streets against Tung's move to pass the anti-subversion bill. It was the biggest demonstration in Hong Kong in more than a decade, rattling the territory's political elite and alarming leaders in Beijing.
Tung has not announced a new timetable for re-introducing the bill, which critics say poses the biggest threat to basic freedoms in Hong Kong since the territory returned to Chinese rule in 1997.
Beijing, which fears Hong Kong will be used as a base to subvert the mainland, has said it wants the law enacted as soon as possible.