The former prime minister's official endorsement, released on personal notepaper, said: "Tireless energy and unquenchable enthusiasm are Jeffrey Archer's hallmarks. These are precisely the qualities that the Conservative candidate for mayor of London requires. Jeffrey has my wholehearted support."
Lord Archer of Weston-super-Mare said on BBC Radio 5 Live he was "immensely proud" to have been endorsed by her and by the former prime minister John Major.
The endorsement came on the same day as a warning for Tony Blair, contained in a Mori poll for The Sunday Telegraph, which showed that Labour could lose the mayoral election unless the leadership allowed Ken Livingstone, the rebel Labour candidate, to run.
The poll put Lord Archer in second place, at 23 per cent, to front-runner Mr Livingstone, Labour MP for Brent East, at 40 per cent. If Mr Livingstone did not run, the poll suggested Lord Archer would win on 31 per cent with Glenda Jackson, a former transport minister, getting 28 per cent for Labour.Reuse content