The decision contradicts a promise of no change in dress code when the country declared itself an Islamic republic last month. Female civil servants will be expected to use a head tie and wrap their hair.
The U-turn was made in a presidential directive to public sector employees, effective from December 31, 2015.
The memo states: "All female staff within the government ministries, departments and agencies are no longer allowed to expose their hair during official working hours...Female staff are advised to use head ties and neatly wrap their hair."
The decision was greeted with shock by civil rights groups after President Yahya Jammeh earlier pledged that there would be no change in dress code for women. That promise was made in mid-December when hedeclared the roughly 90-percent Muslim majority country an Islamic republic.
Change of mind
"Gambia's destiny is in the hands of the Almighty Allah," Jammeh said on TV last month as the country became the second in Africa after Mauritania to renounce secularism.
But then the former army officer immediately added: "I have not appointed anyone as an Islamic policeman. The way women dress is not your business."
Several Muslim countries deploy religious police to enforce sharia rules regarding morality. But many police forces have faced severe criticism for the types of punishment meted out to those caught violating religious codes, which includes flogging.
Gambia's opposition have accused Jammeh of violating the country's constitution both in last month's announcement and in the new rules for female workers.
The presidential order, which did not create exceptions for non-Muslims, ordered that all department heads implement the new policy.
The dress code requirement follows stricter rules introduced against the media and homosexuality since Jammeh took power in a coup in 1994.
Jammeh's government has been branded one of the world's most oppressive regimes. Human Rights Watch said his paramilitaries and secret police have tortured and murdered hundreds of people.
Last year, Gambiaexpelled the EU's envoy to the country
after Brussels cut its foreign aid to Banjul over its human rights record.
mm/msh (AFP, dpa, Reuters)