Councils in Gwynedd, Ceredigion and Powys awarded the work to Bam, Carillion, Laing O’Rourke, Willmott Dixon, Kier and Vinci.
The award has sparked fury among local firms who have failed to win a place on the lucrative framework.
Chris Wynne, the managing director of Wynne Construction in Denbighshire, warned that the whole framework system was threatening the future of regional contractors who often miss out to the majors.
He told the Daily Post: “These procurement strategies are spreading across local authorities and within the Welsh health estate and hospitals.
“We aren’t getting past the first hurdle. We are being excluded and never get a chance to get to the table.”
Wynne has now written to economy minister Ieuan Wyn Jones.
The letter states: “Clearly you do not appreciate the scale of the impact of these procurement strategies and awards will have on the indigenous Welsh companies.
“When these large non-Welsh framework contractors pack up and leave there will be no sustainable legacy in terms of training or long term employment.
“Procurement bodies here in Wales are failing to wake up to this fact, ultimately leading to further redundancies and job losses.”
Glyn Watkin Jones, chairman of Bangor-based contractor Watkin Jones said: “Whatever money these big companies make out of these projects goes out of Wales.
“The Assembly Government should be supporting local construction companies.
“But we have the ridiculous situation now where 40% of our workforce is working in Scotland where we have secured six major contracts at £15m a piece. Yet we can’t get on a school list for £5m in Wales.”
Plaid Cymru AM for Aberconwy Gareth Jones said: “I have nothing against competition from outside. But it’s a one-sided competition and we can’t carry sufficient capacity.”
Huw Williams, head of Gwynedd council’s consultancy department said: “Such arrangements do not preclude the council from undertaking open tender processes for any individual project, and this is the option which the council usually follows.”
Powys council said: “We require successful contractors to use local businesses where possible. We also ask them to support an apprentice scheme for local people.”
A spokesperson for the Welsh Assembly Government said: “By law all construction projects have to be advertised on a EU wide basis and the contractors we choose provide the best value for the Welsh taxpayer’s pound.”