Ben Wallace, the former British defence secretary, described him as a “sub-committee council leader” who was unfit to run Germany, adding that Putin was “laughing at him”.
Reports that Atacms missiles had been authorised emerged in The New York Times on Sunday, and were not confirmed by US officials.
The issue dominated the first day of the G20 summit in Brazil, where Sir Keir met Xi Jinping.
He is the first prime minister to meet a Chinese premier in six years.
Downing Street refused to say whether they discussed the issue, but said they each promised to “engage honestly and frankly on those areas where we have different perspectives, including … Russia’s war in Ukraine”.
Some allies of Donald Trump condemned Mr Biden’s decision to allow long-range missiles inside Russia, although none of the officials he has nominated in key roles have yet criticised the move.
Donald Trump Jr said: “The military industrial complex seems to want to make sure they get World War 3 going before my father has a chance to create peace and save lives.”
It is unclear whether Mr Trump would allow Atacms and Storm Shadows to be used after he takes office on Jan 20.
He has promised to facilitate talks between Putin and Volodymyr Zelensky before he takes office, and end the war through peace talks.
Separately, members of the AUKUS alliance of countries announced on Monday that they would accelerate development of hypersonic missiles by coordinating testing, with up to six trilateral test flights expected by 2028.
Hypersonic weapons have been described as the future of missile warfare, but US development has been slow. The UK is expected to develop its own hypersonic capability by 2030.