It is illegal for mainland law enforcers to operate in semi-autonomous Hong Kong under the city's mini-constitution, the Basic Law.
But there are already concerns that Chinese operatives are working undercover after the alleged abductions of a city bookseller and a reclusive Chinese businessman.
The rail link plan comes at a time when fears are worsening that Hong Kong's freedoms are under threat from an ever more assertive Beijing.
The high-speed connection out of the harbourfront West Kowloon station is set to open in 2018, linking to the southern Chinese city of Guangzhou 80 miles (130 km) away and then onto China's national rail network.
A proposal backed by the Hong Kong government's top advisory body Tuesday would see mainland border staff control a special immigration zone at the Hong Kong terminus.
There are already numerous transport connections between Hong Kong and the mainland, but Chinese immigration checks are done on the other side of the border.
City leader Carrie Lam insisted the new checkpoint arrangement was not a breach of the Basic Law and was designed to cut travel time.
"The crux of the matter is really to find a means that is legal to support this convenience for the people of Hong Kong," Lam told reporters.
Pro-Beijing lawmaker Priscilla Leung said such joint immigration areas were common around the world and that Hong Kong would be "leasing" the portion of land at the terminus to China.
But opponents say the new plan is a clear breach of the Basic Law and another sign that Hong Kong is being swallowed up by China.
Veteran lawyer and democracy advocate Martin Lee, who helped draft the Basic Law in the 1980s, said creating an exception within Hong Kong where mainland Chinese laws are enforced would set a "dangerous precedent".
It would put at risk the semi-autonomous "one country, two systems" set-up guaranteed when Britain handed Hong Kong back to China in 1997, Lee told AFP.
The government wanted to force through the plan to make Hong Kong people "feel closer to Beijing, the sovereign power", added opposition legislator Claudia Mo.
The plan now needs approval from the city's legislature, which is weighted towards the pro-China camp.
The rail link is one of a number of cross-border infrastructure projects, including a new bridge to the mainland and the neighbouring casino enclave Macau.
Passengers from Hong Kong could reach Beijing in under 10 hours on the new line, but controversies have plagued the project, with snowballing costs now at HK$84.42 billion ($10 billion).
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)