Richmond Park MP Zac Goldsmith said reports that he was flirting with Ukip were a “100 per cent non-story”. However, the eurosceptic MP did not dispute that he had said he “might be tempted” if the anti-EU party had a better environmental policy.
Mr Kelly, the son of a wealthy businessman, gave no explanation for his departure.
The 36-year-old was only elected in 2010 and had a majority of 3,856. But his constituency was identified by academics as one of those where Nigel Farage’s anti-EU party was likely to perform well next May, making his chances of being re-elected more slender.
Asked before his announcement whether he was planning to follow Mr Carswell into Ukip’s ranks, Mr Kelly said: “I won’t be joining Ukip, though I wish Douglas Carswell all the best.”
Fellow Tory MP Conor Burns said: “Very disappointed my colleague Chris Kelly will not contest his Dudley seat at the next election. We are losing the wrong people.”
Ukip MEP Patrick O’Flynn said: “This is yet further evidence that David Cameron is losing the confidence of his MPs.
“Chris Kelly has apparently become convinced that he is unlikely to retain his seat standing in the colours of Cameron Conservatism.”
Labour claimed “confidence in the Prime Minister is collapsing even inside his own party”.