The UK government has hit out at reports that a British-Iranian labour rights activist has been given a sentence of 10 years in Tehran for participating in an outlawed group.

A Foreign Office spokesperson said in a statement on Friday that London “strongly” condemned the sentence handed out to Mehran Raoof, a former teacher from north London.

“We continue to do all we can to support Mehran and his family, and continue to raise his case at the most senior levels,” they added.

The response follows an announcement on Wednesday on Twitter by Iranian lawyer Mostafa Nili that both Raoof and a German-Iranian woman, Nahid Taghavi, a retired architect, had been sentenced to 10 years for membership of an illegal group and eight months for anti-government propaganda.

The sentencing of both Iranian dual nationals comes against a backdrop of deteriorating relations between Britain, its western allies and Iran.

On Friday, the G7 group of economically advanced nations accused Iran of orchestrating a drone strike on an Israel-linked tanker that claimed the lives of a former British soldier and a Romanian national.

The United States, Britain and Israel had already pointed the finger at Iran over the attack on the MV Mercer Street off the coast of Oman.

Iran has strongly denied having any link to the attack, which came as tensions grow in the region and talks to revive the 2015 deal on the Iranian nuclear programme remain at a standstill.

The cases of the dual nationals was seen as a way to increase pressure during the recent talks on the future of the deal. Iran regards its nationals arrested for breaches of US sanctions as state hostages in the same way the west regards its dual nationals as being picked up purely to increase Iran’s bargaining leverage in the nuclear talks.

The families of dual nationals, including British-Iranian woman Nazanin Zaghari Ratcliffe, have accused Tehran of using their loved ones as pawns in a wider geopolitical standoff with the West.

Earlier this year, Amnesty International called for Raoof’s unconditional release, saying he had been arbitrarily detained in Tehran’s notorious Evin prison.

The human rights monitor said he was a “prisoner of conscience”, who had been helping to translate English-language news articles and discussing workers’ rights in Iran, where trade unions are banned.