The home secretary, Theresa May, has ordered that the Chinese dissident artist Ai Weiwei should be given a full six-month visa to visit Britain and sent him a written apology after personally intervening in the case.

The artist was originally granted a visa for only 20 days after UK Home Office officials ruled that he had failed to disclose a criminal conviction on his application form. Ai was detained in China for 81 days in 2011 but was never charged with any offence.

A Home Office spokesman 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 is due to visit London in September for his first major institutional exhibition in Britain at the Royal Academy of Arts, whose director has described the original decision as “unspeakably unfortunate”. The landmark solo exhibition has a private preview on 15 September and the truncated visa meant that he would have been able to attend the opening but might not have been able to supervise its installation.

The 20-day business visitor visa he was granted would have run out just before China’s president, Xi Jinping, is due to start a high-profile state visit in October.

Ai made his visa situation public by posting a letter from the visa section of the British embassy in Beijing stating that his entry to the UK had been restricted because he had failed to declare his “criminal conviction”. The letter said it was a “matter of public record that you have previously received a criminal conviction in China, and you have not declared this”.

Ai said he had never been charged or convicted of a crime and had unsuccessfully attempted to clarify during several telephone conversations with the UK visas and immigration department and the Beijing embassy.

He only had his passport returned to him last week after it was confiscated by the Chinese authorities four years ago during a crackdown on political activists. He was held over alleged crimes of bigamy and tax evasion but was released without charge.