An Iranian-British academic detained in Iran was arrested for spying, according to Iranian news agencies, quoting an "unidentified informed source".
Abbas Edalat, a professor at London's Imperial College, was reportedly detained on 15 April while attending an academic workshop in Tehran.
The Foreign Office says it is "urgently looking into" reports of his arrest.
Imperial College said it was "understandably concerned" for Prof Edalat's welfare.
According to the agencies the source described Prof Edalat as part of a "network" of British spies whose members have been identified and arrested.
So far the Foreign Office has not commented on the spying allegations.
But the Center for Human Rights in Iran - which first highlighted Prof Edalat's arrest - condemned what it called Iran's "continued arbitrary arrests of dual nationals without transparency and lack of due process".
The group says he was detained by the Iranian Revolutionary Guard.
BBC Persian TV correspondent Kasra Naji said it was "ironic" that Prof Edalat, who regularly travelled to Iran and was a regular commentator on Iranian state media, had been a founder of an anti-war protest group that opposed sanctions.
"The arrest of Professor Edalat indicates that talks between Iran and Britain to sort out a host of outstanding issues may have hit a dead-end," he said.
Iran detains British-Iranian academic. Imperial college Professor was the founder of an anti-war group opposing sanctions and military intervention in Iran! pic.twitter.com/gv60sXaT1V— Kasra Naji (@BBCKasraNaji) April 25, 2018
A spokesperson for Imperial College London said: "Professor Abbas Edalat is a valued colleague who has worked for Imperial College London since 1989.
"He is a professor of computer science and mathematics who has made many important academic contributions throughout his career.
"We are understandably concerned for his welfare following reports of his arrest in Iran and we are urgently seeking further information."
There are nearly 30 dual nationals in jail in Iran - among them several dual British-Iranian nationals.
One of them - Nazanin Zaghari Ratcliffe, 39 - is currently serving a five-year jail sentence after being convicted of spying charges.
Mrs Zaghari-Ratcliffe, who was arrested at Tehran Airport on 3 April 2016, has denied the charges against her.
BBC diplomatic correspondent James Landale said the latest arrest comes amid growing tensions between Iran and the US as the UK and other European powers try to stop both sides pulling out of the deal curbing Iran's nuclear capability.