The revised budget is nearly four times the £2.4bn estimate when London’s bid succeeded less than two years ago.
Alan Ritchie, general secretary of Ucatt, said: “The Olympics will only be delivered on time and on budget, if the Olympic Delivery Authority (ODA) quickly makes the decision to only use direct employed labour.”
“The ODA must learn the lessons from the Wembley stadium fiasco where a reliance on a subcontracted workforce led to chronic delays, confusion and spiralling costs.
“We should follow the model of Heathrow Terminal 5, which used direct labour, and is on course to be delivered on time and on budget.
“If the ODA fails to heed these lessons then taxpayers’ money will disappear into a financial black-hole.”
A direct labour model will also allow for the creation of a substantial number of apprenticeships and will help to reduce the chronic skills shortages in the British construction industry, he added.
Ritchie is in intensive negotiations with the ODA to agree what kind of employment model should be adopted for the Olympics.
The union claims the talks are currently deadlocked.