Sybil (c. 1999 – 27 July 2009) was a cat living at 10 and 11 Downing Street who was employed as the chief mouser to the Cabinet Office as the pet of the chancellor of the Exchequer, Alistair Darling, and his wife, Margaret.

When introduced in September 2007 Sybil was the first cat employed at Downing Street as chief mouser since Humphrey, who retired in November 1997.

[2][3][4] She was moved from the chancellor of the Exchequer Alistair Darling's family residence in Edinburgh to 11 Downing Street in the summer of 2007,[5] and employed as chief mouser to the Cabinet Office in September of that year.

[12] Upon her arrival The Daily Telegraph stated that those who had written to the Treasury concerning Sybil's welfare had received a "souvenir photo" of her in return.

[13] On 13 September 2007, two days after her introduction,[14] the former prime minister Margaret Thatcher met Sybil during a visit to 10 Downing Street.