The selection of the new "charge d'affaires ad interim" did not require the consent of the British government, a foreign ministry spokesperson told Reuters, which first reported the move earlier on Friday.
More than 900 people opposing the junta have been killed by security forces since the coup, drawing international condemnation and sanctions including from Britain.
"The consent of the receiving State is not required," the spokesperson said in a statement, citing the Vienna Convention on Diplomatic Relations. The statement did not name the new appointee.
A spokesperson for the military-controlled government in Myanmar did not respond to calls from Reuters seeking comment.
The Myanmar Accountability Project, a UK-based rights group, said the appointee for the London job was Htun Aung Kyaw, who served as a fighter pilot during a long army career. A source familiar with the matter also said Htun Aung Kyaw was Myanmar's new pick, but Reuters could not confirm that.
In a statement this week the Myanmar Accountability Project urged Britain not to recognise the representative appointed by the military saying it would be "a gross double standard and a moral outrage".
The former ambassador, Kyaw Zwar Minn, was locked out of the London embassy in April after calling for the release of detained civilian leader Aung San Suu Kyi.
Kyaw Zwar Minn remains in Britain and has urged the British government to refuse to recognise any envoys appointed by the junta and to send them back to Myanmar.
Britain has imposed sanctions on members of Myanmar's military and some of its business interests following the coup, and has called for democracy to be restored.
The military junta said it took power because elections last year were fraudulent, an allegation rejected by the electoral commission.
Britain on Friday appointed a new ambassador to Myanmar, Pete Vowles, who previously worked in diplomatic and international development roles in Africa and Asia. - Reuters