In a letter to May's Downing Street office, Chris Patten said Hong Kong was facing "increasing threats to the basic freedoms, human rights and autonomy" that its people were promised at the 1997 handover to China.
The letter by Patten and Liberal Democrat lawmaker Paddy Ashdown urged May to "go on insisting on the continued validity of the Sino-British Joint Declaration and the principles of 'one country, two systems'".
Patten has repeatedly spoken out about the importance of protecting its democratic freedoms.
Since 1997, Hong Kong has been ruled under a semi- autonomous "one country, two systems" deal which allows rights unseen on the mainland, but Beijing is increasingly tightening its grip.