Harriet Yeo, a former president of the TSSA transport union, an NEC member for eight years and chairwoman in 2012/13, is the most senior Labour figure to leave the party in favour of Ukip, in what will be seen as a blow to Mr Miliband.
Writing in the Daily Telegraph, Ms Yeo said she had become “increasingly disillusioned” at the Labour leader’s refusal to offer an in/out referendum on EU membership.
She claimed to have been told that “a majority” of the shadow cabinet want the party to offer a public vote, but had been ordered to keep quiet on the issue.
Ms Yeo said: “Once in a generation there is a pivotal moment in a country’s history. I believe we have reached that moment.
“It is time to decide whether we remain in the EU. The only party I trust to offer us that choice is Ukip.”
Ms Yeo said she would not be joining Ukip and insisted her resignation was not “a swipe” at Mr Miliband.
“He is a principled leader for whom I have great respect,” she said. “His view is that a referendum is not the right step for our country. I just see it differently. I have to follow my conscience.
“I have therefore decided to support Ukip in the next general election. With very deep regret, I will be renouncing my Labour party membership with immediate effect.”
Ukip leader Nigel Farage said: “I’m delighted that Ukip can now count upon the support of such respected figure as Harriet Yeo.
“A life-long trade unionist and Labour Party member who served as chair of Ed Miliband’s NEC, she is yet another voice calling for Britain to have a choice about its future. We are welcoming support from across the board and Harriet’s support is evidence of this”.
Formerly the leader of the Labour group on Ashford Council in Kent, it is understood Ms Yeo will now sit as an independent.
Labour sources said that Ms Yeo was removed as group leader last week after being accused of non-attendance at meetings and a failure to undertake casework, and was deselected on Monday as a candidate for the 2015 local elections.
A Labour Party spokesman said: “The vast majority of the Labour Party are united behind our position on Europe, believing Britain’s best interests are served by staying in a reformed EU and not sleepwalking towards an exit which would cost British jobs and influence.
“The truth is Ukip are a party of Tory people, Tory policies and Tory money: they are more Tory than the Tories.”