UPDATE 1-Japan's Dec machinery orders cast doubt on strength of capex

(Adds details on forecasts)

* Dec core machinery orders -11.9 pct m/m vs forecast -2.3 pct

* Jan-March core machinery orders seen +0.6 pct

* Data casts doubt on outlook for capital investment

TOKYO, Feb 15 (Reuters) - Japan's core machinery orders tumbled in December and companies expect orders to rise only marginally in January-March, raising concerns that recent gains in capital expenditure will start to peter out. The 11.9 percent decline in core orders in December was more than the median estimate of a 2.3 percent fall and follows a 5.7 percent increase in November. The machinery orders come a day after gross domestic product data for October-December showed capital expenditure had risen for five consecutive quarters and suggest this run of gains may not continue. Companies surveyed by the Cabinet Office forecast that core orders, which exclude those for ships and from electric power utilities, said they expect orders to rise only 0.6 percent in January-March after a 0.1 percent decrease in October-December. Orders from manufacturers tumbled 13.3 percent month-on-month in December, after a 0.2 percent decline in the previous month because of reduced orders from makers of metals and manufacturing equipment. Non-manufacturers' orders fell 7.3 percent, following a 9.8 percent gain in November due to declines in orders from retailers and wholesalers. For January-March, manufacturers expect their orders to fall 5.7 percent, while non-manufacturers expect their orders to rise 7.4 percent. Since taking office in late 2012, Japanese Prime Minister Shinzo Abe has used tax breaks and other incentives to encourage more domestic capital investment. Companies were slow to respond, at first, but in recent years several indicators have shown consistent gains in capital expenditure. Some companies are also spending on technology to deal with chronic labor shortages caused by Japan's shrinking and rapidly aging population.

(Reporting by Stanley White; Editing by Eric Meijer)