Contractors and consultants have already been suffering disruption as key staff leave their posts. Keith Down, 47, an engineering consultant at Parkman, has been told he will probably be called up.
He said: "Parkman has been fine about the call-up but I'm leaving there next week to join Atkins, so I have had to pre-warn Atkins that I may have to go war."
Regional and smaller contractors have also been affected. Rennie Construction and Plant, which is based in Monmouth in south Wales, has lost its general manager, who has been called up to join his Territorial Army regiment.
The firm's owner, Rob Rennie, said he had lost two key staff and two more were due to leave in days. He said: "I have briefed my clients that there may be delays to projects. The call-ups have hit us badly."
Rennie said the news had been sprung on him very quickly and left him no time to plan for the situation. He said: "My workers were literally contacted by phone on Monday to tell them to be ready for a call-up and then received letters the very next day ordering them to join their regiments."
Contractors will be required to hold open the jobs of any workers called-up until the war is over and they can return home.
Firms have seven days to apply to a government officer to have a call-up notice deferred or revoked. The only way that contractors can annul them is to persuade a government adjudicator that the loss of the employee would cause serious harm to the business.
I have briefed clients about delays. The call-ups have hit us badly
Rob Rennie of Rennie Construction and Plant
Contractors will not be required to continue to pay reservists normal salaries, as they will receive pay from the Ministry of Defence.
There are 46,000 people in the volunteer reserve forces, of which 23,000 are in full-time employment. It is estimated by the Construction Confederation that of the 23,000 reservists that work full-time, up to 5% work in construction.
Jerry Lean, the Construction Confederation's industrial relations director, said employment regulations stipulated that the MoD could make payments to cover certain costs incurred as a result of the call-up.