The dissident revealed on Thursday that the officials in Beijing would only issue him with a three-week visa after claiming he had failed to declare a "criminal conviction" on his application.

Mr Ai posted a photo of the letter from the British Embassy on his Instagram account which said it was "a matter of public record that you have previously received a criminal conviction in China, and you have not declared this".

Chinese authorities only returned Mr Ai's passport last week and he is coming to the UK in September for a show of his work at London's Royal Academy of Arts.

In 2011, Mr Ai was detained for 81 days amid a crackdown on government critics, but not formally charged with a crime.

A Home Office spokesman claims Home Secretary Theresa May was not consulted after Mr Ai was initially refused a visa.

He said: "The Home Secretary was not consulted over the decision to grant Mr Ai a one-month visa.

"She has reviewed the case and has now instructed Home Office officials to issue a full six-month visa.

"We have written to Mr Ai apologising for the inconvenience caused."

Mr Ai is famous for designing Beijing's Olympic stadium and filling Tate Modern's Turbine Hall with ceramic sunflower seeds.