Iran carried out a cyber attack on British lawmakers earlier this year, The Times newspaper reported on Saturday, citing an intelligence assessment of the incident.
The report came the day after Britain joined other European countries in warning the United States against harming a nuclear deal with Iran.
Britain's parliament was hit by a "sustained and determined" cyber-attack in June, designed to identify weak email passwords, just over a month after a ransomware worm crippled parts of the country's health service.
The Times said that the attack was Iran's first significant cyber-attack on a British target after the hack was initially blamed on Russia.
Britain's National Cyber Security Centre was not immediately available for comment.
The Iranian government had no immediate comment.
While the motive for the attack has not yet been established, the hackers were not seeking simple financial profit, The Times said.