Official documents discussing the Convention of Peking, the document signed 99 years ago today that gave Britain a lease over the New Territories, are still considered too sensitive for release by the authorities in London.
Although it took effect on July 1, 1898, the document now denounced by Beijing as an 'unequal treaty' was signed on June 9.
The 10-paragraph treaty also created the Kowloon Walled City no-go area by allowing Chinese officials 'to continue to exercise jurisdiction' in Kowloon City.
But when local academic Professor Peter Wesley-Smith formally applied to London earlier this year for 20 records from 1930s relating to the treaty he was given access to only three - even though he wanted to view them after the handover.
'There's still sensitivity - or it may just be the British obsession with secrecy,' said Professor Wesley-Smith, of the University of Hong Kong's Department of Law, who is updating his book Unequal Treaty 1898-1997.
At first classified for 50 years, documents had been reclassified for another 25.
He believed the documents withheld related to the Walled City.