A former Conservative Party chair has switched sides in Britain's upcoming European Union referendum, accusing campaigners promoting an exit from the 28-nation bloc of peddling hate and xenophobia.

Sayeeda Warsi said Monday that moderate voices in the "leave" campaign have been drowned out.

Warsi is one of the most prominent Muslim politicians in Britain. She said her decision was sparked by a poster depicting a crowd of migrants walking through Europe with a warning in capital letters that said: BREAKING POINT.

Warsi told the BBC that "this kind of nudge-nudge, wink-wink xenophobic racist campaign may be politically savvy or politically useful in the short term, but it causes long-term damage to communities."

Campaigners for "leave" were bemused by her decision, saying they weren't even aware she had been a supporter.