Fitch was the only one of the world's three largest ratings agency to provide the Islamic Republic with a rating for its debt.

Governments need independent ratings for their sovereign debt so that potential investors can evaluate how much risk they are taking on.

The Iranian government issued its last sovereign debt in 2002. It finished paying back the €325m ($513m) Eurobond on 21 April, prompting Fitch to withdraw its rating on 24 April.

Any new bonds are unlikely in the near future, due to international sanctions targeting Iran’s controversial uranium enrichment programme. Investment banks are increasingly unwilling to act as lead arrangers for the country’s sovereign debt.

“The problem is that you do not know what the government is going to use the money for,” said a spokesman for Fitch.

The only remaining sovereign rating for Iran is the BB- rating from Capital Intelligence.