LONDON, Jan 31 - OPEC oil output in January registered the biggest monthly drop since July, a Reuters survey found, as several members implemented new voluntary production cuts agreed with the wider OPEC+ alliance and unrest curbed Libyan output.

The Organization of the Petroleum Exporting Countries (OPEC) pumped 26.33 million barrels per day (bpd) this month, down 410,000 bpd from December, the survey found. December's total strips out Angola, which has left OPEC.

The Reuters survey aims to track supply to the market and is based on shipping data provided by external sources, LSEG flows data, information from companies that track flows - such as Petro-Logistics and Kpler - and information provided by sources at oil companies, OPEC and consultants. (Reporting by Alex Lawler Additional reporting by Ahmad Ghaddar Editing by David Goodman )