Foxconn has raised spending in the past three years to build factories, and competitors Quanta Computer, Pegatron and Wistron followed. That inland migration is fuelling mainland development, with economic growth in Sichuan and Henan outpacing that of Guangdong as the government seeks to reduce income disparity in the face of a workforce decline.

"Closer to the pool of workers has always been one of the major reasons," Foxconn spokesman Louis Woo said, "Henan and Sichuan have always been the largest sources of migrant workers. That was why we moved to both of these provinces to tap their labour pool."

Wages in Sichuan and Henan have surged 120 per cent in six years because of economic growth, increasing local competition for labour and slower population growth nationwide. That threatened to dent the migrant workforce that Guangdong factories relied on, even as Foxconn, Quanta and Compal Electronics geared up for sales that were likely to grow a combined 30 per cent up to 2015, Waral said, citing consensus estimates.

Capital expenditures at Foxconn's Taipei-listed flagship, Hon Hai Precision Industry, surpassed US$6 billion during the past three years, according to company filings.

Foxconn employed 300,000 people in Henan in the fourth quarter of last year, compared with almost none in 2010. The Sichuan and Chongqing workforce has jumped to 150,000 from almost none in the same period. The company employs 400,000 people in Guangdong.

The government has also spurred investment in inland regions through its "Go West" policy, which is designed to help curb regional differences in economic development. Growth in Sichuan and Henan will probably outpace Guangdong's for a third consecutive year this year, according to Nomura.

Foxconn began operations in Shenzhen in 1988 as the communist government used the region as a test bed for efforts to ease economic restriction.

The drive lured migrant workers to Guangdong, boosting its population by 66 per cent in the two decades to 2010. The province is now the country's most populous and has the largest economy, government statistics show.

Manufacturers also are looking inland as a slower birthrate dents the mainland's workforce. The working-age population fell by 3.45 million last year to 937 million, according to National Bureau of Statistics data. That was the first decline in "quite a long period", agency head Ma Jiantang said in January.

Differences in average wages between inland provinces and Guangdong have more than halved in the past six years, driving the two locales toward labour-price parity, according to an analysis of workforce, wages and labour demand. Wages in Chongqing, have surpassed those in Guangdong, it shows.

Migrants accounted for 99 per cent of workers at Foxconn's two Shenzhen facilities producing Apple products, according to data from the Fair Labour Association, which was hired by the US company last year to investigate conditions among its suppliers. About 14 per cent of workers at its Chengdu factory were migrants, it said.

"Manufacturers are spurred to move inland because that's where most of their employees come from and are likely to be the source of their workforce in coming years," he said.

"Faster rising wages inland have the effect of both reducing possible incentives for migrants to travel thousands of miles for work, and forcing wages higher as employers compete for labour."