The defection of David Campbell Bannerman to the Conservative Party has been labelled a "wretched defection" by UKIP MEP Derek Clark.
Mr Clark, MEP for the East Midlands, joined leading UKIP figures in slamming the decision, saying it betrayed the party and voters in the Eastern region who voted for Mr Campbell Bannerman.
And Mr Clark pointed to interviews given by Mr Campbell Bannerman only last year when he claimed the Prime Minister could not be trusted on Europe.
He said: "Mr Campbell Bannerman's defection is a gross betrayal of UKIP.
"He has betrayed those on the party list in the Eastern region who did not get elected because he was top of the list.
"He has betrayed the officers and members in that region who worked so hard to get him elected.
"And worst of all, he has betrayed those who voted UKIP, thinking they were getting a real opponent of our EU membership. The electorate will take note and Eastern region members will have to work even harder to overcome this handicap next time round."
Mr Clark said: "In 2010, Campbell-Bannerman wrote a trenchant criticism of David Cameron and the Conservatives in the UK, accusing them of hypocrisy."
In the interview, Mr Campbell Bannerman said: "Over the years observers in Brussels have learned first-hand how the Conservatives talk ‘Eurosceptic’ at home but behave quite differently when they arrive in the Belgian capital to carry out their parliamentary work."
He added: It's overwhelmingly clear now that trusting Cameron with Europe will be as misguided as trusting Blair on Iraq."
Mr Campbell Bannerman even said the British people faced higher prices all because the Conservative and Liberal Democrat coalition did not have the stomach to either get the country’s rebate back or suspend EU payments until it had its accounts signed off, adding, "it's like Blair never left."
Mr Clark added, that as an MEP, the party’s position was weakened by what he called Campbell Bannerman’s "selfish actions".
He said: "Having started with 13 MEPs we took the lead in forming the EFD group, which enables the party leader to speak for the group.
"This privilege means that, in important debates, our UKIP leader is in the first round to address Parliament, after Jose Manuel Barosso and/ or Herman Van Rompuy who often then withdraw, leaving it to ‘other speakers’.
Mr Clark offered his total support to the party in the Eastern region and pledged he would continue to work for UKIP, both in Brussels and elsewhere.
He said: "I condemn this wretched defection but it makes no difference to whatever efforts I am able to make in the name of UKIP. "
