A key measure of China's manufacturing activity dropped to its weakest level in more than three years in January, data showed Monday, underlining weaknesses in the world's second largest economy.

The official Purchasing Managers' Index (PMI), which tracks activity in factories and workshops, fell to 49.4, figures from the National Bureau of Statistics showed.

It was the lowest figure since 49.2 in August 2012, and was below a medium forecast of 49.6 in a Bloomberg survey of economists.