MEP Roger Helmer has defected from the Conservative Party to the UK Independence Party (UKIP).
The East Midlands MEP is expected to explain his reasons at the UKIP spring conference in Lincolnshire on Saturday.
But he told BBC Radio 5live that his new party was more in tune with the concerns of Tory voters.
"UKIP better represents the views of Conservative voters than David Cameron's Conservative Party," he said, on a wide range of issues.
"Take Europe, take climate change and energy, take immigration.
"On all these issues, UKIP presents the sort of policies that Conservative voters believe in and David Cameron's Conservative party sadly does not."
Mr Cameron had given away too many powers and too much money to Europe, he said.
Europe had in the last few years become a huge issue due to the euro crisis and the bailouts, said Mr Helmer.
But it was not just about Europe, he said - the spread of wind farms, for example, was another major concern.
"Conservative voters instinctively know what the right thing is but unfortunately the Conservative Party doesn't."
UKIP leader Nigel Farage said: "A senior MEP from the Conservative Party, like Roger Helmer, defecting to us sends a message that people are taking UKIP very seriously."
European parliamentary rules state that when an MEP resigns there is no by-election, but the next in line from an agreed party list fills the vacancy.
Mr Helmer, 68, had been planning to step down in December and hand over to his friend and fellow Eurosceptic, Rupert Matthews.
Mr Helmer had been due to quit on 31 December over disagreements with Conservative policies but later indicated he would not do so until the position over his successor had been clarified.
It is understood the Conservative Party had raised objections to Mr Matthews, the author of a number of books on alien encounters and UFOs.
It now appears Mr Helmer has decided to stay in office - his term runs until 2014 - but switch allegiances.