Al-Shabab has banned Islamic Relief, one of the few remaining aid agencies able to operate in the region of Somalia controlled by the armed group.
The fighters said on Monday that they had "officially revoked Islamic Relief's permit to work" in the areas they control, according to messages posted on the group's Twitter account, which were authenticated by al-Shabab officials.
Al-Shabab said the Islamic Relief group was being banned because it "has repeatedly failed, despite the persistent warnings, to comply with the operational guidelines" established by the group.
"Islamic Relief was also found to be covertly extending the operations of banned organisations, particularly WFP," it said.
Somalia, ravaged by nearly uninterrupted civil war for the past two decades, is one of the most dangerous places in the world for aid workers, despite being one of the regions that needs them the most.
While the fighters have lost control of a string of towns in recent months to African Union troops and Somali government soldiers, they still control large parts of rural southern and central Somalia.
Last year al-Shabab ordered shut several UN agencies including the World Food Programme (WFP) as well as other international aid agencies.
Islamic Relief, a British-based aid agency, has worked in Somalia since 1996.
The group's projects focus on "improving access to water, healthcare and education for pastoral communities and people who have fled their homes due to conflict, floods or drought," according to its website.
Somalia was the Horn of Africa country worst hit by a harsh drought last year that left about 12 million people in dire need of relief aid, with several areas in the country declared famine zones.
Islamic Relief was not immediately available for comment on Monday.