LONDON — A British-Iranian woman who has been imprisoned in Tehran has begun a new hunger strike to demand her freedom, her husband said as he vowed to support her by fasting outside the Iranian Embassy in London.

The woman, Nazanin Zaghari-Ratcliffe, was a project manager with the Thomson Reuters Foundation when she was arrested in April 2016 at a Tehran airport as she headed back to Britain with her young daughter, Gabriella, after a family visit.

Ms. Zaghari-Ratcliffe, now 40, was sentenced to five years in jail after being convicted of plotting to overthrow Iran’s clerical establishment, a charge strongly denied by her family and the foundation, a charity that operates independently of Thomson Reuters and Reuters News.

“Today I received a phone call from Nazanin in prison,” her husband, Richard Ratcliffe, said in a statement on Saturday. “She had informed the judiciary that she has begun a new hunger strike — she will drink water — to protest at her continuing unfair imprisonment.”