As an experienced pollster, Cook worked for the Labour Party for nearly 30 years, providing electoral analysis during six general elections, including the three that Labour won under Tony Blair.
His knowledge of the UK electoral geography saw him co-ordinate Labour's submission to successive boundary reviews, and introduced geo-demographic analysis into the party’s strategy and campaigning.
He joins Lexington as senior political and polling adviser. Cook will help the team undertake a detailed analysis of the electoral landscape and the key trends and battlegrounds that will shape the outcome.
This work will inform Lexington’s pre- and post-election planning with clients, including likely governing arrangements and the future path for Brexit.
Ian Kennedy, partner at Lexington Communications, said: "It is fantastic to have Greg on board.
"His unrivalled knowledge and insight will add further depth to our outstanding political team as we continue to help our clients prepare for a general election and navigate Brexit."