The decision by the MP for East Wiltshire, southwest England, was revealed at a press conference in central London.

The defection represents a coup for anti-immigrant Reform leader Nigel Farage.

“Kruger will head Reform’s efforts to prepare for government,” Farage said.

Reform now has five seats in parliament, against 399 for the governing Labour Party and 119 for the main opposition Conservatives.

“The Conservative Party is over … it is over as a national party, and over as the principal opposition to the left,” said Kruger, who was considered to be an important voice in developing Tory policy while serving as shadow minister for work and pensions.

“We are planning change on a scale that the system has not seen since the modern civil service was created in the 19th century,” he added.

The defection comes as a blow to Conservative leader Kemi Badenoch, who has been unable to capitalise on the unpopularity of the governing Labour party.

“I hoped after our defeat last year that the Conservative Party would learn the obvious lesson,” said Kruger.

“But no, we’ve had a year of stasis and drift and the sham unity that comes from not doing anything bold or different or controversial,” he added.

Badenoch responded by saying that her party was “not going to get blown off course by these sorts of incidents”.

Reform, a populist hard-right party, leads the ruling Labour and main opposition Conservative party in national polling, although the next general election is still likely to be another four years away.