Patrick Reslow said that he was leaving the Moderate Party because he was unhappy with the 'December Agreement' made with the Social Democrats to prevent the Sweden Democrats from blocking the government budget in 2014.
"My future role as an MP will be carried out within the framework of the Sweden Democrats," he said at a special press conference with the Sweden Democrat party leader, Jimmie Åkesson. "I look forward to working with them in the run-up to the 2018 election."
Reslow, a 47-year-old from Malmö, is a member Constitutional Committee in parliament and as such was the most senior Moderate yet to defect to the Sweden Democrats, who have seen support grow strongly since the last general election.
The Moderate MP Sten Andersson defected to the Sweden Democrats in 2002, but otherwise defections have been on the local level.
At Wednesday's press conference, Reslow said that he had also been disappointed with the Moderate Party's lukewarm approach to the talks opened up with the Sweden Democrats earlier this year.
This shows that the Alliance parties are not interested in being the active opposition parties and the question is whether they are interested in leading the country," Reslow said. "For me, that is serious."
The shock announcement came as the Sweden Democrats' leader Jimmie Åkesson's house was vandalised on Tuesday evening.
"It was the worst attack on the house yet yesterday evening, and this time they wanted (in addition to pelting the facade and car with eggs and firecrackers) to break into the house," Åkesson's partner Louise Erixon wrote on Facebook, according to the Swedish newspaper Expressen.