Ip said his decision was a “pity” but she would accept newcomers more carefully, and not just pick “someone who wants the post of vice-chairman”.

“Every man wants more space. I can fully understand that,” Ip said. Her differences with Tien were “not fundamental”, she added.

Tien’s decision shrinks the party’s presence in the Legislative Council, with only two lawmakers to represent it – Ip herself and political novice Eunice Yung Hoi-yan.

According to Ip, Tien first alienated himself from the party last year, when he moved his office further away from hers in the Legco building following the September polls.

Tien on Monday shied away from naming a particular point of contention with Ip, saying their differences were revealed after “long-term observation”.

“When Ip is highly popular while Michael Tien is also highly popular, people wonder what the NPP is all about,” he said.

Political observers suggested the pair might have clashed over what kind of relationship they should keep with the liaison office, which represents the central government in Hong Kong.

In January, Tien launched a thinly-veiled attack on the office, complaining that the chief executive race had “lost its shape” due to increasing “interference” by an “invisible hand”. But Ip has been reluctant to criticise the office.

Tien said on Monday he would keep a “cordial relationship with distance” with the liaison office.

However, Ip rebutted suggestions that Tien’s departure had anything to do with her attitude towards Beijing’s representatives .

“Tien ought to be even more loyal to the country as he’s a delegate to the National People’s Congress,” she said, insisting Tien knew more mainland officials than she did.

Tien’s departure raised questions about the party’s succession plans, according to Chinese University political scientist Ivan Choy Chi-keung.

“The biggest problem is there’s no one to pass on the torch to, as few would expect Eunice Yung to be able to take the helm,” he said.