Chieh Chung, a military researcher at Taiwan’s National Policy Foundation, said the new route would be about 7km from the median line, which would squeeze the pre-warning and reaction time for Taiwan’s air defences.
“It is trying to completely eliminate and deny the existence of the median line,” he said.
Taiwan’s defence ministry said China’s “rude and unreasonable” actions can easily lead to an increase in tensions.
“For unknown aircraft entering our air defence identification zone (ADIZ), they will be dealt with in accordance with operating procedures and emergency handling regulations to ensure the safety of our airspace,” it added.
The ADIZ is a broad area Taiwan monitors and patrols to give its forces more time to respond to threats, and Chinese military aircraft have not entered territorial Taiwanese air space so far.
China has downplayed the furore.
Its Taiwan Affairs Office described the changes as “routine” to help alleviate pressure on air space, and that China had no need to discuss this first with Taiwan.
Speaking at a regular news conference in Beijing on Wednesday, Chen Binhua, a spokesperson for the office, said the “so-called median line does not exist”.
“The M503 route is for civil aviation and is in the Shanghai flight information region. It is to alleviate congestion for the related airspace and routes, and ensure aviation safety,” he said.
The M503 route is mostly used by Chinese airlines and also by foreign airlines going to and from cities like Shanghai to Southeast Asia.
Flights to and from Taiwan and China’s Xiamen and Fuzhou take a circuitous route skirting the median line, rather than flying directly across the strait.
Taiwan has complained about the M503 route before, in 2018, when it said China opened the northbound part of it without first informing Taipei in contravention of a 2015 deal to first discuss such flight paths.