A former chair of the Welsh Conservative Party and MEP has joined UKIP, claiming the Tories have lost their core vote on several issues.
Beata Brookes said UKIP "are saying what people are thinking".
A North Wales Tory MEP for 10 years, she said attacks on UKIP by senior Conservatives - including the prime minister - such as "fruitcakes" and "clowns" were unwise.
It comes as UKIP hailed early gains in local elections in England.
Counting is also under way in the Anglesey council elections - the only ones in Wales this year - and UKIP also finished second behind Labour in the South Shields parliamentary by-election.
Ms Brookes told BBC Wales she was not surprised by the gains UKIP had made in the English council elections.
"The Conservative Party has lost its core vote on a number of issues and there are now many disillusioned Tories, including me, who've turned to UKIP," she said.
Of the attacks on UKIP, she said: "You don't behave like that in politics.
"It was very rude and disrespectful but at the end of the day it's provided a backlash - it's backfired on them," she said.
"If Mrs Thatcher were Prime Minister now she would have a taken a strong line on Europe and held it. We would not be in the dire straits that we are in now if she were still at the top".
Ms Brookes, who left the Conservatives a year ago, said David Cameron should not have gone into coalition with the Liberal Democrats.
She said they should have "had the courage" to form a short-term minority government in 2010 before calling another general election.
She earned the nickname the Celtic Iron Lady in her 10 years as an MEP between 1979 and 1989.