31 July 2015: Read an update to this story

Last week, Ai Weiwei received his passport, which had been held by the Chinese authorities since he was secretly detained in 2011 over a contested tax bill. Following this, the artist announced his intention to travel to his landmark show at the Royal Academy, due to open in September.

Today, he released a letter received from the UK government in an Instagram post to his 126,000 followers. The letter explains that his entry to the UK had been restricted because of a failure to declare a “criminal conviction.”

Despite his secret detention in 2011, Ai Weiwei has never been charged or convicted of a crime in China.

Instead of a regular 6-month business visa, Ai Weiwei has been granted a 20-day visa to attend the opening of his Royal Academy show.

Reacting to the news, Tim Marlow, the RA’s Artistic Director, said: “We’re concerned that Ai Weiwei has not been granted a six month UK visa by the British government. We hope for a speedy resolution to this situation and we continue to look forward to welcoming Ai Weiwei at the Royal Academy for his first major institutional exhibition in the UK this September.”

Listen to Tim Marlow on BBC Radio 4’s World at One.