LONDON (Reuters) - Britain is working to resolve the case of detained Iranian-British aid worker Nazanin Zaghari-Ratcliffe, foreign minister Boris Johnson said on Tuesday.

Zaghari-Ratcliffe was sentenced to five years in prison in Iran on charges that remain secret, her family said in September.

"The Foreign Office is in regular contact with the Iranian government at all levels," Johnson told parliament. "It remains a matter that is of the utmost priority for this government and we are doing our level best to resolve it."

Zaghari-Ratcliffe works for the Thomson Reuters Foundation, a London-based charity that is independent of Thomson Reuters and operates independently of Reuters News.

Iran's hardline Revolutionary Guards have accused her of trying to overthrow the country's clerical establishment. The Thomson Reuters Foundation and her husband have dismissed the Revolutionary Guards' accusation.

(Reporting by Elizabeth Piper; writing by William James; Editing by Guy Faulconbridge)