HBG has climbed three places to the top spot, thanks to a £25m college teaching facilities project in Birmingham and a £19m office development in Edinburgh.
The Birmingham contract has also helped Davis Langdon & Everest keep its lead in the quantity surveyor's league.
Close behind HBG in the monthly contractors' chart is Laing O'Rourke, with a £35m mixed-use scheme in Newcastle and a £15m residential development in Plymouth.
The firm's slew of contracts worth £57.8m also catapulted it from nowhere to number five in the annual chart this month.
This information is compiled for Building by Emap Glenigan, which analyses an average of 8500 construction projects each week. For details, email Marcus Oughton at firstname.lastname@example.org or call 01202-432121.