Shares in Capita plunged 28.7% to 678.8p, a four-year low, after it warned full-year profits could be as low as £535 million, well below forecasts of £614 million.
The company said in July that clients were delaying project decisions following the UK's vote to leave the European Union, and said in today's trading statement those difficult conditions were persisting.
'Our performance in the second half of the year to date has been below expectations, as a result of a slow-down in specific trading businesses, one-off costs incurred in the Transport for London congestion charging contract and continued delays in client decision making,' it said.
The news represents a big blow to Citywire AA-rated fund managers Woodford and Barnett, who are among the company's biggest shareholders.
Today's share price plunge has wiped around £80 million off the £9.4 billion Woodford Equity Income fund, and a combined £120 million from the £11.6 billion Invesco Perpetual High Income and £5.9 billion Income funds.
Woodford had been buying more of the stock after it took a big hit from the Brexit vote. The stock had been among his best performers as it mounted a recovery from those Brexit lows until today's plummet.
Robin Speakman, analyst at Shore Capital said he feared 'more negative news to come' from Capita and placed his 'hold' stance under review.
'We see this as the effects of a hiatus from the Brexit referendum - likely continuing for several quarters yet, in our view. However, we have also commented for some time over the sustainability of operating margins in an evolving environment with even greater competition from more capable peers,' he said.