HELSINKI, Aug 8 (Reuters) - Finland's Olkiluoto 3 nuclear reactor resumed test production on Monday after months of maintenance, data from operator TVO showed, raising hopes that the much-delayed unit will enter regular service later this year.

Under construction since 2005, the 1.6 gigawatt (GW) OL3 was originally meant to start operation in 2009 but has faced several technical mishaps which sparked costly delays and a lengthy legal battle. (Reporting by Essi Lehto, editing by Terje Solsvik)