Turkey's president Recep Erdogan is on a three-day visit to the United Kingdom where he will meet the Queen and prime minister Theresa May.

It comes as the British government is trying to secure a post-Brexit trade deal with Ankara. The UK was the first country to send a minister to visit Turkey following a failed military coup against Erdogan in July 2016.

His visit will include a joint press conference with May, a speech at the thinktank Chatham House, and a meeting with the Queen at Buckingham Palace.

Christian persecution charity Open Doors warned Turkey was among the most dangerous countries in the world to be a Christian. A spokeswoman said that improving religious freedom was in both Erdogan's and May's interest.

'The meeting of world leaders is always a good opportunity for the British government to promote its values,' the spokeswoman said. 'Open Doors would always encourage the UK government to discuss human rights issues, especially the international right to freedom of religion or belief, with foreign counterparts.

'Turkey is ranked at 31 on the 2018 Open Doors World Watch List of countries where it is most dangerous to be a Christian. Evidence indicates that a focus on religious freedom can be extremely favourable for business and trade.

'An improvement in Turkey's record in this area is in both the UK and Turkey's interest.'

Liberal Democrat leader Vince Cable criticised the visit as a whole.

'The UK has a strong, proud history of democracy and human rights, but our reputation on the world stage is in danger of being eroded by this Conservative government's desire to woo world leaders like [Donald] Trump and Erdoğan,' he said. 'May's administration appears to have substituted diplomacy for sycophancy in its pursuit of Brexit.

'By permitting a state visit and audience with the Queen, May and [foreign secretary] Boris Johnson are essentially rolling out the red carpet for a man with a disregard for human rights, who is responsible for alarming oppression and violence.'

US pastor Andrew Brunson is currently on trial accused of collaborating with supporters of Fethullah Gülen, the Muslim cleric blamed for the failed 2016 coup against Turkey's President Erdogan. It's believed that Brunson has been detained as a bargaining chip to secure the release of Gülen from the US, where he is now based.

He has already been jailed for the past 19 months, and after two Izmir court hearings now awaits a third on July 18. At the close of the second hearing Brunson's lawyer, Ismail Cem Halavurt, sharply criticised the 'outlandish' use of secret witnesses against Brunson, saying no real evidence against the pastor had been presented.

Erdogan has rejected pleas from US president Donald Trump and religious freedom ambassador Sam Brownback to release Brunson.