Taiwan will sign a key investment pact with China in top-level talks in Taipei on Thursday amid protests from anti-Beijing critics against closer economic ties with the former foe.

China's chief negotiator Chen Yunlin and his Taiwanese counterpart Chiang Pin-kung will put their name to the much-awaited deal, providing a legal umbrella for Taiwanese businesses operating in China.

They will also sign a cooperation pact to speed up the time needed for products to clear customs in the hope of boosting bilateral trade.

"The pacts will promote bilateral investment, offer protection to corporations and the large number of Taiwanese businessmen based on the mainland," Chiang said.

The new pacts follow the sweeping Economic Cooperation Framework Agreement (ECFA) in 2010 that eased tariff restrictions and gave trade a major push. But some Taiwanese fear they will strengthen China's hold over the island.

Protesters have been tailing Chen since his arrival on Wednesday, shouting slogans opposing the investment pact and claiming Taiwan's independence from the mainland.

Taiwan and China split in 1949 at the end of a civil war, and remained implacable enemies for decades, even after the island's businesses started exploring opportunities across the strait.

However, ties have improved markedly since Ma Ying-jeou became Taiwan's president in 2008 on a Beijing-friendly platform, leading to the signing of 16 agreements between the two sides to boost trade and civil exchanges.

China is Taiwan's largest trade partner with more than 80,000 Taiwanese companies operating on the mainland, where they have invested more than $100 billion over the years.

Thursday's meeting is the eighth in four years between Chen and Chiang -- unthinkable a decade ago in the absence of formal diplomatic ties but now almost routine.

But this meeting could be their last, with observers expecting the 70-year-old Chen to retire as China's top negotiator after Chinese President Hu Jintao transfers his power to deputy Xi Jinping this year and next.

"Chen is way too old to stay in that position, and he has pretty much accomplished his mission," a former Taiwanese official familiar with China affairs told AFP, speaking on condition of anonymity.

"Besides, Beijing would need another negotiator who is more familiar with the issues of culture and politics as it seeks bilateral agreements in those areas to broaden its exchanges with Taiwan."