The proposed structure at Royal Mint Court, near the Tower of London, would be the largest embassy in Europe, covering 20,000 square metres.
China blasted the UK government on Wednesday for its latest delay in deciding whether to approve the construction of a huge new embassy in London.
British authorities said on Tuesday that a planned decision by 10 December would be pushed back to 20 January following mounting security concerns.
"The UK's repeated delay in granting approval is completely unjustified and the reasons they cited are untenable," Chinese Foreign Ministry spokesperson Lin Jian said.
The government is "strongly dissatisfied," he added.
The plans for the embassy close to London's financial district and sensitive data cables have been stalled for years.
The proposed structure at Royal Mint Court, near the Tower of London, would be the largest embassy in Europe, covering 20,000 square metres.
Critics have raised concerns that the building would be used as a base for espionage and lawmakers from across the political spectrum have urged the government to reject the proposal.
The UK government's planning agency said the decision had been postponed to allow more time to consider it.
"The Home Office and Foreign Office have provided views on particular security implications, and have been clear throughout that a decision shouldn’t be taken until we affirm that those considerations have been completed or resolved," Tom Wells, a spokesperson for Prime Minister Keir Starmer, said on Tuesday.
Critics have suggested the decision was delayed amid preparations for Starmer's expected visit to Beijing early next year.