All 22 lawmakers from the pan-democratic camp on Monday issued a joint letter to Chief Executive Carrie Lam, demanding she reveal the details of the West Kowloon Station deal she signed with Guangdong authorities on Saturday.
Under the planned arrangements, part of the terminus for the high-speed rail line to Guangzhou will be put under mainland control. Critics say this amounts to Hong Kong giving away part of its territory and the plan will be unconstitutional.
One of those who signed the letter, the Civic Party's Tanya Chan, accused Lam of a complete lack of transparency by refusing to make public the details of the agreement.
Chan told RTHK's Priscilla Ng that the government must also explain the legal basis of the arrangements, and said that if there isn't one, the implementation process has to be stopped immediately.
"The government disclosed very little information to the public and it's very difficult for legislators to discuss this co-location arrangement... I think she should respect Hong Kong people," Chan said.
The deal signed with Guangdong requires the approval of the National People's Congress Standing Committee and local legislation will also be needed for it to be put into practice.