Image copyright EPA Image caption The artist had his passport returned by the Chinese authorities last week

The Home secretary is "looking into" the case of dissident Chinese artist Ai Weiwei, who has been refused a six-month visa by the British government.

Ai has been accused of lying on his visa application form.

The artist posted a letter on Instagram stating his entry to the UK had been restricted because he failed to declare his "criminal conviction".

But Ai, though detained in China for 81 days in 2011, has never been charged or convicted of a crime in China.

The artist was granted a 20-day visa to attend the opening of his London show.

However, it may mean he cannot supervise the installation of the landmark solo exhibition at the Royal Academy of Arts which has a private preview on 15 September.

A Home Office spokesperson said: "This case did not come to the Home Secretary, and she is now looking into it."

Ai's letter, from the visa section of the British embassy in Beijing, stated Ai's entry to the UK "has been restricted to the requested dates of travel... because you have failed to meet the business visitor rules".

Image copyright Instagram Image caption The letter from the British Embassy in Beijing claims the artist failed to declare a criminal conviction

The document, signed by an entry clearance manager from the UK Visas and Immigration department, continued: "It is a matter of public record that you have previously received a criminal conviction in China, and you have not declared this.

"While an exception has been made in this instance, any future application you submit must be completed as accurately as possible," the letter concluded, adding the artist might otherwise face a 10-year ban if he did not comply.

In a separate post on Instagram, Ai stated he "has never been charged or convicted of a crime" and had "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. This decision is a denial of Ai Weiwei's rights as an ordinary citizen," he added.

Last week, Ai had his passport returned to him after it was confiscated by authorities four years ago.

It was taken when he was arrested in 2011 during a government crackdown on political activists. He was held over alleged crimes of bigamy and tax evasion, but was released without charge.

Ai was fined 15m yuan ($2.4m, £1.55m) for tax evasion in a civil case in 2012. The artist lost an appeal against the fine - which he maintains was politically motivated in retaliation for his criticism of the Chinese government.

Image copyright Instagram Image caption The artist posted an image of his 20-day visa on Instagram

In a statement, Britain's Home Office said visa applications were considered "on their individual merits and in line with the relevant legislation".

It added: "Mr Ai has been granted a visa for the full duration of his requested dates of travel".

Ai's 20-day visa means he will not be in the UK when China's President Xi Jinping make a high-profile state visit in October - potentially avoiding any diplomatic embarrassment at a time when Britain is trying to improve relations with China.

The BBC has asked the Home Office to clarify what criminal conviction it believes Ai has, but has yet to receive a response.

Tim Marlow, artistic director at the Royal Academy of Arts, 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."

Amnesty UK's James Savage said: "Whilst on this occasion this could be an administrative error that we hope will likely be swiftly rectified, Ai Weiwei's case does speak to a wider problem.

"The Chinese authorities routinely charge and convict human rights defenders with spurious, politically motivated criminal offences, for precisely the reasons this case illustrates, to tarnish an individual's reputation and make it difficult for them to travel freely and raise their concerns internationally."

Analysis - Martin Patience, Beijing correspondent

It was only this month the Chinese authorities returned Ai Weiwei's confiscated passport after holding onto it for almost four years.

The dissident artist was clearly delighted at the prospect of travel and did not hang around when it came to applying for visas.

It probably came as a bit of shock then when he was accused by a British official of lying on his visa application form.

His detention in 2011 caused an international outcry, but he was never actually charged or convicted of any crime.

That fact appears to have escaped the British bureaucrats processing his visa.

The UK embassy in Beijing has yet to comment on whether it considers Ai a convicted criminal.

This may well turn out to be nothing more than an embarrassing bureaucratic blunder.

But the visa decision means that the artist will not be in the UK when China's President Xi Jinping arrives for a state visit in October.

The artist is a well-known critic of the ruling Communist party.

And rightly or wrongly, some of his supporters will see this as a UK attempt to curry favour with Beijing.