The husband of Nazanin Zaghari-Ratcliffe, detained for more than five years in Iran, has gone on a hunger strike again after she received another sentence.

Richard Ratcliffe started his protest action on Sunday outside the UK Foreign Office in central London.

He is camped in a tent outside the building's main entrance to pressure the government to secure the release of his wife and other detained dual British-Iranian nationals.

"Nazanin is threatened with prison any time soon so we're here just to make the point that the government needs to change its approach. I've had some meetings behind closed doors with the government, pushing them to do something different. They haven't been willing to, so here we are," Ratcliffe said.

Zaghari-Ratcliffe served five years in prison after being taken into custody at Tehran's airport in April 2016 and convicted of plotting the overthrow of Iran’s government, a charge that she, her supporters and rights groups deny.

In May, she was sentenced to an additional year in prison on charges of spreading “propaganda against the system”.

Ratcliffe went on a 15-day hunger strike two years ago outside the Iranian Embassy, a move he credits with getting their seven-year-old daughter Gabriella released.

Rights groups accuse Iran of holding dual-nationals as bargaining chips for money or influence in negotiations with the West, something Tehran denies.