Leading Conservative Baroness Warsi has switched from supporting Leave to Remain, accusing some Brexit campaigners of spreading "hate and xenophobia".
The former Foreign Office minister said the final straw was a Ukip poster showing migrants queuing up to get into Europe under the slogan "Breaking Point".
Lady Warsi told The Times: "That 'breaking point' poster really was - for me - the breaking point to say, 'I can't go on supporting this'.
"Are we prepared to tell lies, to spread hate and xenophobia just to win a campaign? For me that's a step too far."
The newspaper originally had "defects" in its headline but later changed it to "walks out".
Vote leave say they were unaware that she ever backed their campaign.