A fourth Iranian man has been arrested as part of a counter-terrorism investigation in London.

The Metropolitan Police said the 31-year-old was detained in north-west London on Friday morning under the National Security Act.

Three other men – all from Iran and aged 39, 44 and 55 – were arrested in London on 3 May as part of the investigation.

Police have obtained warrants to hold those three men until 17 May.

The investigation is separate to one involving an alleged plot to target the Israeli embassy in London, for which four other Iranian men are still being questioned, while a fifth has been released on bail.

Police said searches were carried out at two addresses in north-west London on Friday.

All four men have been detained under section 27 of the National Security Act 2023, which authorises police to detain those suspected of “foreign power threat activity”.

On Tuesday, Home Office minister Dan Jarvis described the two separate investigations as “some of the largest counter-state threats and counter-terrorism actions that we have seen in recent times”.

He also confirmed the men were the “first Iranian nationals arrested under the National Security Act”.

Iran was the first country to be listed at the top level of the foreign influence registration scheme (Firs), which aims to protect the UK from malign foreign influence.

Anyone who is directed by the regime to carry out activities in the UK must declare it or face five years in prison once it comes into effect in July.