Iran's former president, Mohammed Khatami, arrives in Scotland today for a visit opposed by Iranian émigré groups and a group of MPs who charge that thousands of students were imprisoned during his rule.
However, the Metropolitan Police last night said that there were "insufficient grounds" to pursue a criminal investigation against Mr Khatami, after two exiled Iranians said that they had been tortured by his government.
A police spokeswoman said: "Having considered the content of the letter and the supporting evidence provided, we consider there are insufficient grounds to make an arrest and there is not sufficient basis for a criminal investigation."
Mr Khatami is the highest ranking Iranian official to visit the UK since the 1979 Islamic revolution, and will be awarded an honorary doctorate by St Andrews University this evening.
His staff say the five-day visit, on a "purely personal" basis, is aimed at defusing the mistrust between Tehran and the West which has deepened over Iran's nuclear ambitions.
After opening a new Iranian studies institute and library at St Andrews, Mr Khatami will travel to London, where he will speak at the Chatham House think- tank and meet leaders of the Anglican Church.Reuse content