Citigroup on Wednesday introduced a “poison pill” that discourages investors from buying more than 5 per cent of its shares and deters large shareholders from raising their stakes in an effort to protect a $43bn tax benefit.

Citi’s surprise move came as the company launched its long-awaited $58bn conversion of preferred shares into common stock which will leave the US government with a 34 per cent stake and help the bank bolster its battered balance sheet.