China's vast army of migrant workers continued to expand last year but the rate of their pay increases slowed, a government report showed, as growth in the world's second-largest economy decelerated.

The movement from China's countryside to its booming cities over recent decades has been one of the greatest human migrations of all time.

The number of migrant labourers totalled 262.6 million in 2012, up 3.9 percent from the previous year, a survey by the National Bureau of Statistics (NBS) found.

The average monthly income of those who worked outside their home towns rose 11.8 percent year-on-year to 2,290 yuan ($374), it said -- but the rate of the increase was down 9.4 percentage points from 2011.

In 2012 China recorded its slowest economic growth for 13 years at 7.8 percent.

Labourers from central and western China increasingly preferred to work close to their origins with growing job opportunities in those regions and high living expenses in the more developed coastal areas in the east, the NBS said Monday.

The survey of nearly 200,000 migrants workers across the country found their average age climbing to 37.3 last year, compared with 36.0 in 2011 and 34.0 in 2008, it added.

China's ageing population is an increasingly important demographic issue, with the labour pool falling last year for the first time in decades.