Residents, rights groups and China hawks have opposed the development of the embassy complex, which would be the largest in Britain if built, fearing it could be used for spying and harassing dissidents.

A decision has been delayed for years, with housing minister Steve Reed extending the deadline on Tuesday from December 10 to January 20.

China's foreign ministry spokesman Lin Jian said the delay was "completely unreasonable" and the reasons for it "entirely untenable".

"China is deeply concerned and strongly dissatisfied" with the delay, he told a news conference, adding that the application to build the embassy was submitted long ago.

Beijing bought the site, which once housed the Royal Mint, in 2018 for a reported $327 million.

A spokesman for British Prime Minister Keir Starmer suggested on Tuesday that "particular security implications" meant that more time was needed to consider the planning application.

The decision is viewed as a key issue in Starmer's attempts to reset relations with China, which remain strained by accusations of spying and by tensions over the fate of Hong Kong, a former British colony.

The Chinese embassy in London accused Starmer on Tuesday of "groundless accusations" after he said China posed a national security threat to Britain in a speech a day earlier.