"All of Iran's attempts to threaten, silence, intimidate, harass, harm or coerce people on our territories through proxies or other means are unacceptable," reads a joint statement also backed by France, Germany, Canada and Sweden, among others.
The statement comes after an incident at RAF Fairford in late September, when several people were arrested on suspicion of plotting terrorist attacks.
British Prime Minister Andy Burnham reiterated in Parliament on Monday that there are "strong indications" Tehran was involved in the incident.
Iran “categorically rejects the accusations again,” the state news agency IRNA reported on Monday evening, citing Ali Moussavi, the country’s ambassador to London.
