At the same moment, Mr Sharon's long-time deputy, Ehud Olmert, formally took on the title of acting Israeli prime minister.
Mr Olmert won last month's general election as head of the Kadima party which was formed last year by Mr Sharon, and is in the process of forming a new coalition government.
Under Israeli law, a prime minister's incapacitation is deemed permanent after 100 days and his title and powers are revoked for good.
Mr Sharon, 78, has been in a coma in a Jerusalem hospital since suffering a massive haemorrhagic stroke on January 4.
He underwent several brain operations in the initial days of his hospitalisation but never regained consciousness.
Mr Olmert, who was vice premier at the time of Mr Sharon's stroke, immediately assumed his powers and was named interim premier.
At a special session last Tuesday, the Israeli cabinet voted unanimously to designate 60-year-old Mr Olmert the acting prime minister.
The vote was brought forward because of the week-long Passover holiday which began on Wednesday.
The title change from "interim" to "acting" does not alter Mr Olmert's powers of office.
Mr Olmert, whose Kadima party won the most parliamentary seats in Israel's March 28 general election, will take the full-fledged title of prime minister in coming weeks if the new government he is currently forming is sworn in as expected.
Mr Sharon is expected to be moved soon to a long-term care facility, or back home to his ranch in southern Israel under medical supervision.