The civil servant is currently one of five chief press officers in Number 10, reporting to head of news Vickie Sheriff.
McAllister is the second senior Downing Street comms official to sign for Tesco in recent years. She follows in the footsteps of Tom Hoskin, a former Number 10 head of news, who made the switch in 2009. Hoskin is now the firm’s head of group media.
Britain’s biggest supermarket chain also employs Tony Blair's former private secretary David North as UK corporate affairs director and Blair’s former head of events Victoria Gould as government affairs manager.
In the Number 10 press office, McAllister specialises in home affairs. Before joining Number 10, she held comms posts at the Department for Culture, Media and Sport, and at the Department for Work and Pensions.
At Tesco, McAllister will take a newly created role running the UK press office. She will report to UK comms director Tom Curry, who joined earlier this year from the Olympic Delivery Authority, where he was head of media.
Curry confirmed the appointment, saying: ‘Ruth will be responsible for leading UK media relations for Tesco and will bring first-class expertise from her time on the front line of government communications. We are looking forward to her starting with the team early next year.’