The United Nations nuclear watchdog declared on Monday that it had reached a “dead end” in its year-long effort to get Iran to clarify whether it covertly researched the development of a nuclear weapon.

Unveiling one of the most damning reports it has yet published on Iran’s nuclear programme, the International Atomic Energy Agency said it had been unable to make “any substantive progress” in its attempt to get Iran to answer allegations that there may have been a military dimension to its nuclear programme.