A Taiwanese democracy activist has been detained in China and accused of “endangering national security” in the latest sign that President Xi Jinping’s crackdown on dissent is growing ever wider.

The detention of Lee Ming-che, who disappeared 10 days ago after crossing into the mainland from the semi-autonomous Chinese territory of Macau, will further complicate the already-tense relationship between China and self-governing Taiwan, which Beijing claims is its territory.

Mr Lee, who worked at a college in Taipei, the Taiwanese capital, has been co-operating with civil society activists in China for several years but travelled to the mainland on this occasion to arrange medical treatment for his mother-in-law, according to Amnesty International, the human rights campaign group.