China has quietly more than doubled its deployment of mainland security forces in Hong Kong, according to foreign envoys and security analysts, in the most dramatic move yet by Beijing to prepare for a potential worsening of unrest in the global financial center.

Last month, Beijing moved thousands of troops across the border into this restive city, which has been wracked by protests since June. The state news agency Xinhua described the operation as a routine "rotation" of the low-key force China has kept in Hong Kong since the city's handover from Britain in 1997.

A month on, seven Asian and Western envoys have told Reuters they are certain the late-August deployment was not a rotation at all, but a reinforcement. Three of the envoys said the number of Chinese military personnel in Hong Kong had more than doubled since the anti-government protests began in June. They put the number of Chinese military personnel at 3,000 to 5,000 in the months before the reinforcement, and estimated it was now between 10,000 and 12,000.