The UK is preparing to formally declare that Iran's Islamic Revolution Guard Corps (IRGC) is a terrorist organisation.
The legal change would mean it becomes a criminal offence in the UK to belong to the group or support its activities.
Whitehall sources said no announcement was imminent and many details remained to be sorted out.
But they said it was "broadly correct" to say the government intended to proscribe the IRGC.
The proscription - first reported by the Daily Telegraph - would follow a similar decision made by the US in 2019.
And it would mark a further hardening of the UK's position towards Iran after intelligence agencies said the country posed a direct threat, citing 10 plots against British or UK-based individuals over the last year.