Aggreko said it could be hit by the scaling back of the Nato military presence in Afghanistan as well as weakening economic growth in emerging markets.
Analysts predicted that the company, which employs more than 400 people in Dumbarton, will scale back production.
Shares in Aggreko, long a stock market darling due to its rapid expansion at a time of high energy demand, fell 461p to 1664p, cutting its stock market value to £4.5bn from £5.7bn.
After what was the second disappointing stock market update from Aggreko in three months, its share price has given up all the gains it had made in the previous 15 months.
The stock is also 31.1% below the high of 2415p reached on September 17.
Aggreko told investors: "After a year of strong growth in 2012, the economic environment we will be facing in 2013 is particularly uncertain in many of our markets and it is difficult at this stage to provide a definitive view of the likely pattern of trading in 2013."
But it added: "We currently believe that group performance in 2013 is likely to be slightly lower than in 2012."
It was already known that Aggreko would not have an event on the scale of the Olympic Games in 2013. London 2012 was worth £50 million of revenues to the company.
But it said that "the planned reduction in numbers of US troops in Afghanistan will lead to a further reduction in military revenues".
It is also uncertain as to whether Japanese customers who took up its services in 2011 following the earthquake and tsunami will extend their contracts into the second half of 2013.
Aggreko said: "Our current assumption is that revenues from these three items combined will be around £100m lower in 2013 than in 2012."
On top of this Aggreko believes weakening economic growth in many emerging markets could hit demand for the services of its international power projects arm which constructs temporary power stations in countries facing electricity shortfalls.
The impact could be felt by Aggreko's workers in Scotland.
Aggreko, which has a large production facility in Dumbarton, has earmarked £150m in capital investment for the first half of 2013.
While analysts thought that Aggreko was always unlikely to spend as much as the £420m it committed in 2012, they are tipping the company to make further cuts to investment. It is not clear whether there is likely to be job losses.
Aggreko chief executive's Rupert Soames did not respond to a request for comment.
Aggreko previously warned in October that 2012 profits would be weaker than expected. Analysts have pencilled in even worse earnings for 2013.
Stockbroker Panmure Gordon cut its target price for the stock from 2159p to 1935p.
Panmure analyst Paul Jones wrote in a note for clients: "We now expect 2013 to be below 2012 – something not seen at Aggreko for some time."