Traditionally the chancellor and prime minister live next door
Chancellor Gordon Brown is to move his family into the flat above 10 Downing Street, the Treasury has confirmed.
The move has the full agreement of the prime minister's office and is being made on the advice of Special Branch for security reasons, a spokesman said.
Mr Brown and his wife Sarah, who have two young sons, have lived in a Westminster apartment for nine years.
The chancellor has had the use of the flat at Number 10 since 1997 but has mainly used it for entertaining.
The Treasury spokesman said no building work was required and three rooms currently used as offices would return to being bedrooms.
He said: "With the new addition to their family and having taken the advice of Special Branch, Gordon, Sarah and the boys will be starting to live full time in the current flat at Number 10 which the chancellor has used since 1997.
"As a result, three rooms currently used as offices have been converted back to bedrooms as they were under Margaret Thatcher and John Major.
"This work has been done at minimal expense with no building work involved, all with the agreement and help of the prime minister's office."
Prime ministers have traditionally lived in the private flat on the second floor but the Blair family live in the larger flat above Numbers 11 and 12 Downing Street.
Sir Robert Walpole was the first prime minister to live in Number 10 when he moved there in 1735.
And No 11 Downing Street has been the official home of the Chancellor of the Exchequer since 1828.
The two houses, including the offices below the apartments, are said to be valued at £23.6m.