Canadian engineering consultancy WSP had offered 35p per share for Sweett, valuing it at £24mln at the time, which the latter recommended to shareholders.
But Sweett withdrew that recommendation after receiving a 42p per share cash offer from international asset management and construction consultancy Currie & Brown Holdings Limited.
WSP, formerly known as Genivar Inc, later said its 35p bid was final. It said it had now lapsed the offer, but reserved the right to return to the fray if Currie & Brown pulled out.
Net losses for the year to March were £19.5mln after hefty charges related to restructuring, legal action and disposals as chief executive Douglas McCormick, who joined last year, cleaned up the business.
Sweett was fined £2.25mln in February related to alleged bribery offences that took place in the United Arab Emirates in 2010.