Writing in The Independent Ms Hamnett, who turned up to a reception held by Margaret Thatcher in 1984 wearing a T-shirt declaring "58 per cent don't want Pershing", denounces the Government as "undemocratic" and says Labour's support for "throwing away our independence" has prompted her to join the Tories.
"Cool Britannia is really Craven UK plc, about to be swallowed in a European takeover," she writes.
It is a swingeing critique of the party she once voted for. One of the most politically aware designers, an avid environmentalist and human rights campaigner, she says she is a "true European" opposed to the kind of integration Europe's politicians envision.
"When people call Euro-sceptics Little Englanders, they don't realise there is such a thing as little pro-Europeans too, hidebound by the codes and taboos of an out-dated project," she writes.
Ms Hamnett's arguments will articulate the misgivings held by those unwilling to identify themselves with the Blimpish Eurosceptic brigade, but who are unconverted to the wisdom of the European federalist project.
"The European Union is undemocratic. If Labour wanted Britain to have fuller participation or influence culturally, economically and politically in Europe, they would realise that our main obstacle is not our currency but our nationally poor language skills."
Ms Hamnett, who has lived in several European countries and speaks French and Italian, is given to making dramatic gestures, but her radicalism has usually been of a leftish hue.
Apart from the famous anti-missile protest against Baroness Thatcher, she has sent models on to catwalks with T-shirts bearing slogans about acid rain, rainforests and, more recently, denouncing the human rights record of the Chinese government.
The latter provides another spike in the designer's argument, as she accuses New Labour of bowing to big business instead of pursuing a human rights agenda in China: "everything is for sale", including British independence, she says.
How long the Conservatives will welcome their strange new bedfellow is uncertain, though: apart from the Europe issue, the Tories and their cool new member seem as curiously matched as, say, a Hamnett boiler suit and a Dior jacket.
Why I joined the Tories, Review, page 4Reuse content