The Cabinet Room was a room inside 10 Downing Street where the Cabinet of the United Kingdom was in session. (TV: World War Three) It was the largest room in the building, located on its north-west side. (PROSE: The Time Travellers' Almanac)
Weekly Cabinet meetings were held in the Cabinet Room from 1796. (PROSE: The Time Travellers' Almanac)
The Emergency Protocols to deal with alien incursions were kept in there. (TV: Aliens of London)
If the Cabinet in session was in danger, the four walls of the Cabinet Room became — according to the Ninth Doctor — "about the four most safest walls in the whole of Great Britain" when in 1991, three inches of steel lined every single wall with no way for intruders to get in or the Cabinet to get out. The original Cabinet Room was blown up when Mickey Smith fired the Harpoon missile UGM-84A at 10 Downing Street to kill the Slitheen family. The room had a small, but strong cupboard for the Ninth Doctor, Rose Tyler and Harriet Jones to survive and "ride [...] out" the missile strike by standing under the cupboard's doorframe. (TV: World War Three)
The Cabinet Room was rebuilt along with Downing Street by 2008, in time for the election of the Saxon Master as Prime Minister of the United Kingdom.
The Master's first action was to gas the Cabinet to death while in session, accusing them of being traitors who abandoned their political parties to jump on his bandwagon, while wearing a gas mask to protect himself and mock his victims. (TV: The Sound of Drums)