The Enquirer understands that site staff were called Tuesday night and told to attend a meeting Wednesday morning where they were given the bad news.
One site manager said: “We were all called-up last night and told to go to a meeting at the head office this morning.
“We were given the bad news and all the sites were shut-up at eleven today.”
The company is now in the hands of administrator BDO.
One subcontractor said: “They owe me about 70 grand for a job we are doing in the City.
“I started getting calls this morning and now the site has shut up and we’re told they are in administration.”
No-one was answering the phones at any Killby & Gayford offices this afternoon as the Enquirer tried for an official comment.
Latest results show that for the year to December 31 2010 Killby & Gayford had a turnover of £78m and made a pre-tax profit of £1.9m.
It was projecting turnover of £90m for 2011 after winning several hospital contracts, including a £20m extension for the Holly House Private Hospital in Essex and the Royal Marsden Hospital.
The builder was one of the oldest firms operating in London and the south east and invested heavily in training for its general building and specialist refurbishment contracts.
It employed around 300 staff at its Billericay joinery works and offices in Cambridge, London Clapham and Leeds. They are now waiting for an official statement to hear their fate.
Chief executive Christopher Chivers led the management buyout of Killby & Gayford Group in September 2007.
It is understood the builder was due to renegotiate its £3m overdraft facility next month.
One rival said: “This is a real shame. They were one of London’s oldest building firms and put a lot back into the industry.
“It’s more evidence of just how bad the downturn in the building market is.”