Balfour was down 18%, or 51.6p, to 234.2p, as Andrew McNaughton resigned from the group with immediate effect, following the disclosure of a £300 million shortfall in its UK construction arm.
The wider stock market struggled for direction, with the FTSE 100 Index down 8.4 points to 6814.
Cheer over an improvement in the US services sector was held back by disappointing Chinese manufacturing figures and ongoing anxiety over Ukraine.
Meanwhile the pound surged to highs not seen since 2009 following data showing a better-than-expected April performance for the dominant services sector, prompting speculation that a rise in interest rates may be brought forward.
Sterling was up a cent against the greenback to nearly 1.70 US dollars while it was steady against the single currency at just under 1.22 euros.
In London, Balfour's woes saw it lead the FTSE 250 fallers' board by some distance
Chairman Steve Marshall, who appointed Mr McNaughton as chief executive at the start of 2013, will run Balfour for an interim period.
He said: "Today's trading update is once again disappointing. The board is committed to rapidly addressing the root causes."
In the top-flight, Barclays was down nearly 5% after posting a fall in earnings for the first quarter.
Adjusted pre-tax profits were down 5% to £1.69 billion as earnings from its investment bank slumped by half, driven by a decline in fixed income, credit and commodities revenues.