(Updates with market open prices)

By Ragini Mathur

Dec 18 (Reuters) -

Canada's main stock index fell for the fifth consecutive

session on Wednesday, hurt by commodity-linked stocks, as

investors adopted a cautious stance ahead of the U.S. Federal

Reserve's monetary policy decision.

The Toronto Stock Exchange's S&P/TSX composite index

dropped 0.2% to 25,070.07, trading near a four-week

low.

The U.S. central bank is widely expected to

reduce

rates by 25 basis points at 2 p.m. ET on Wednesday, with

market participants keeping a watch on the Fed's summary of

economic projections (SEP) and comments from Chair Jerome Powell

for clues on the pace of future rate cuts.

U.S. rate cuts generally benefit commodity prices,

particularly precious and base metals, which could boost

Canada's resource-heavy stock market. However, with a rate cut

almost fully priced in, any hawkish tilt could lift the dollar

and put pressure on commodity prices.

"The markets are mostly trading cautiously ahead of the

interest rate decision," said Douglas Porter, chief economist at

BMO Capital Markets.

"Markets have been under downward pressure (in the

recent weeks) for a variety of reasons like the worry about the

potential U.S. tariffs, fairly weak economy and now some

political uncertainty as well."

Prime Minister

Justin Trudeau's

government was thrown into chaos on Monday when his finance

minister and deputy prime minister Chrystia Freeland quit

abruptly. Trudeau, trailing in the polls, faces calls from

within his own caucus to resign.

Among sectors, materials index and gold

miners dropped about 0.7% each, as precious metal

prices retreated ahead of the Fed's policy announcement.

In single stocks,

Nuvista Energy ( NUVSF ) fell 2.7%, after the oil and natural

gas firm said that it temporarily curtailed production in the

greater Wapiti area due to unscheduled maintenance at a

third-party gas plant.