At a specially held press conference this morning the backbencher announced he would stand down as MP for Clacton and fight under the UKIP banner in a by-election for the seat.
Kept under wraps, the news took many journalists by surprise and has since leapt to the top of the news agenda.
The fact that Ukip kept the Carswell defection secret is impressive— Tim Shipman (@ShippersUnbound) August 28, 2014
But it wasn’t just the journalists taken unawares. Explaining his decision, the backbencher said he wanted to "shake up" the Westminster "clique", adding that he had not told Prime Minister David Cameron about the decision.
Jo Tanner, director at InHouse Communications, called the defection a "coup" for the party "which would have caught everyone on the hop".
Tanner was guarded about how damaging the move would be to the Conservatives but praised UKIP’s timing.
"In terms of surprise factor and grabbing the headlines they’ve achieved what they set out to achieve. The fact it was held during the summer recess means lobby correspondents won’t have the same number of reports, events and other political stories to cover."
The by-election would further UKIP's aim of keeping the general election focused on the issue of immigration and Europe, Tanner said, adding that the proximity of the vote to conference season would also prove a challenge.
"Having been caught off guard, the other parties are going to have a by-election to fight around conference season, and with the Scottish Referendum taking place there will be an awful lot for them to cope with in terms of resources."
Caught cold by the switch of allegiance, the Conservative press office nevertheless managed to get a line out on Carswell around an hour after the news broke.
In the event, it decided that attack was the best form of defence when it comes to Europe.
"This is a regrettable and frankly counterproductive decision," the statement read.
"As Douglas Carswell said, the only way to get a referendum on the EU is to return a majority Conservative government. The Conservative Party will contest the forthcoming by-election vigorously to make sure the people in Clacton have a strong Conservative voice in this Parliament and the next."
Naturally, the Labour press team also jumped into the fray, retweeting the views of its leader Ed Miliband and deploying shadow cabinet office minister Michael Dugher for a BBC interview on the issue.
Douglas Carswell’s defection is not just a blow to David Cameron. It shows the Tory Party too divided to stand up for hard-working families— Ed Miliband (@Ed_Miliband) August 28, 2014
The commentariat, meanwhile, were largely impressed. The Telegraph's political commentator Peter Oborne called the move "seismic", while the New Statesman's George Eaton spoke of Carswell's "strong personal brand" and Paul Waugh reached for geopolitical analogies to convey the impact.
Farage famously praised Putin re Ukraine. Today Russian tanks roll across the border, as UKIP parks Carswell on Cam's lawn. #spookytiming— Paul Waugh (@paulwaugh) August 28, 2014
And, perhaps unsurprisingly, Twitter threw up its usual array of comedians to satirise the news.
TEXTED DAVE: AVOID iced water challenge. a) You're post curve. b) Hacks and pundits will say you were cooling off your temper. #carswell— SteveHiltonGuru (@SteveHiltonGuru) August 28, 2014