Britain has denied Chinese artist and dissident Ai Weiwei a six-month visa and restricted him to a three-week visit, claiming that he did not declare a criminal conviction in his application.

Ai said he was informed that his six month application had been turned down by a British Embassy official on Thursday, July 29.

One of the most outspoken critics of China's Communist government, Ai was detained by authorities for 81 days in 2011 in a crackdown by Chinese authorities on dissidents. His company was fined $2.4m (£1.5m, €2.2m) the following year after losing a civil legal case against tax authorities.

Critics accused the Chinese government of using the fines to punish Ai for his criticism of authorities. Recently Ai's passport was returned, allowing him to travel abroad for the first time since 2011.

A letter from the British embassy in Beijing posted by Ai on his Instagram page states it is "a matter of public record that you have previously received a criminal conviction in China, and you have not declared this", adding that he had "exceptionally" been granted a three week visa.

The posting continues: "Ai, who has never been charged or convicted of a crime, attempted to clarify this claim with the UK Visas and Immigration Department and the British embassy in Beijing over several telephone conversations, but the representatives insisted on the accuracy of their sources and refused to admit any misjudgement."

It added: "This decision is a denial of Ai Weiwei's rights as an ordinary citizen, and a stand to take the position of those who caused sufferings for human rights defenders."

Hong Kong-based human rights expert Joshua Rosenzweig said he was not aware of any public record of Ai having been convicted of a crime.

"I would like to know what the public record is because I pay attention to these things and it is not ringing any bells," he told the Guardian.

Supporters claim that the ruling means that Ai will not be able to attend the opening of an exhibition of his work at the Royal Academy in September.