The activist, famous for designing Beijing's Olympic stadium and an outspoken critic of China's ruling communist party, had planned on travelling to London for a landmark exhibition of his work.
But after Ai uploaded a picture of the Home Office letter denying him a full visiting permit, the government backed down, insisting Home Secretary Theresa May had not been consulted on the issue.
A spokesman for her department 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."
The artist originally spoke out on Thursday, posting the message he had received from the British embassy in Beijing the previous day.
Officials originally denied him the six-month visa but said Ai would be “exceptionally” granted a 20-day visa for dates in September.
It emerged he had been refused a six-month residence visa to the UK on the grounds of a "criminal conviction", a claim he denied.