Always on the left of the party, his resignation letter to William Hague yesterday revealed a deep unease with what he regards as the new regime's "increasingly right-wing" policies. "After a period of very serious reflection, there is no longer any doubt in my mind that New Labour more embodies the values for which I entered politics," he wrote.
His decision follows his sacking as shadow minister for London because of his support for the Government's plans to repeal Clause 28 legislation, which bans the "promotion" of homosexuality in schools.
A high-flying and long-time party strategist, Mr Woodward entered Parliament at the last election, winning Douglas Hurd's safe seat of Witney in Oxfordshire.
A trustee of Childline, the telephone counselling service for children, he has spoken up for gay rights before, telling MPs that equalising the age of consent was a decision which would be of comparable historic significance to the abolition of slavery and giving women the vote.
In 1998 he was one of only 18 Tory MPs to vote in favour of lowering the age for gay sex to 16.
In the 1997 leadership campaign he backed first Stephen Dorrell, then Kenneth Clarke, and he has supported Clarke and Chris Patten in their pro-European stance.
His departure is a significant loss to the party. This is not simply because of his advertising and campaigning background, but also because he is just the kind of family man the party needs. Married to Camilla Sainsbury, whose father is the former Hove MP and supermarket boss Sir Tim Sainsbury, he has four young children.
Indeed, until he fell out of favour he was considered to embody the youthful and dynamic characteristics that Mr Hague has been so desperate to cultivate in the party he inherited from John Major. He represented the one-nation approach that many in it feel that Mr Hague should adopt.
His departure is part of a relentless series of setbacks that have befallen Mr Hague since the good Euro-election results in July. One party insider said: "Anything like this is bad because it stops the party in its tracks. Woodward's departure is worse because it feeds the public's image that the Tory party is becoming more extreme."
Woodward, who is 41, was born in Bristol and left Cambridge with a double first. During the Eighties he worked for the BBC, progressing from researcher to editor on That's Life! and producing Panorama and Newsnight.
He also worked for Saatchi & Saatchi and in 1991 he was headhunted by Chris Patten to be the party's new director of communications. He went on to mastermind John Major's 1992 election victory, specialising in television and advertising. Together with Saatchi & Saatchi, he was responsible for planning the "Labour Tax Bombshell" campaign in 1991.
An eloquent man and a determined networker, he even managed to squeeze in six months at Harvard as a Kennedy scholar before entering Parliament.
That's Life! presenter, Esther Rantzen, describes Mr Woodward as "an excellent journalist and a friend" who is "very very bright".
"Shaun was always interested in improving the circumstances of people's lives, particularly the least well off. He was responsible for the groundbreaking work we did on transplants, which not only changed people's lives but changed attitudes towards transplants," she said.
"This decision must have been very difficult for him because he has been at the centre of the Tory party machine, and many of his friends, like Chris Patten and Douglas Hurd, are high-profile party figures.
"He will have done this as a matter of principle. Clause 28 may have been the superficial problem but he feels the party has drifted a long way to the right."
Mr Woodward is the third Tory MP to defect to new Labour. Alan Howarth, then Conservative MP for Stratford-on-Avon, switched parties in October 1995 after speaking about his "profound disagreement" with a range of government policies, and is now arts minister.
Peter Temple-Morris, MP for Leominster and a pro-European, resigned the Tory Whip in November 1997 and sat as an independent before defecting to Labour in June last year.
In addition to politics, Mr Woodward has many other interests. He is a director of the English National Opera and the co-author, with Esther Rantzen, of Ben: The Story of Ben Hardwick.