UPDATE 1-Canada Mackenzie review extends to at least April

* Final arguments to be heard in April

* Timing hinges on release of review panel report in Dec

* Approval of massive project has faced repeated delays

(Adds details)

TORONTO, Oct 7 (Reuters) - Canada's energy regulator said it will not hear final arguments in its review of the C$16 billion ($15 billion) Mackenzie Gas Project until at least April, further delaying the potential approval of the massive Arctic gas pipeline.

Canada's National Energy Board (NEB) said on Wednesday the timing would depend on the release of the Joint Review Panel report, expected in December.

The panel, an independent group appointed by the Canadian government, is examining the potential environmental, socio-economic and cultural effects of the project. The NEB is looking at all other issues, including engineering, safety and economic matters.

The 1,220 km (760 mile) Mackenzie line would ship up to 1.9 billion cubic feet of gas a day to the Alberta border from the Mackenzie Delta on the Beaufort Sea coast. There the gas could be routed on existing lines to Canadian and U.S. markets.

The project has suffered numerous delays while regulators held an epic string of public hearings across the huge region.

At one point, the Joint Review Panel report was expected in October, 2008. Environment Minister Jim Prentice told Reuters last December he expected to receive the panel report at some stage between March and May 2009. [ID:nN03351932]

The pipeline's backers are led by Imperial Oil Ltd (IMO.TO), and include Royal Dutch Shell Plc (RDSa.L), Exxon Mobil Corp (XOM.N) and ConocoPhillips (COP.N). ($1=$1.06 Canadian) (Reporting by Jeffrey Hodgson; Editing by Anshuman Daga)