* TSX down 0.4%

* BoC rate decision on June 5

* GFL Environmental jumps to touch near four-week high

June 3 (Reuters) - Canada's main stock index slipped on Monday in a volatile session, led by losses in energy stocks tracking lower oil prices, while investors awaited the Bank of Canada's interest rate decision and other economic data due this week.

At 10:19 a.m. ET (14:19 GMT), the Toronto Stock Exchange's S&P/TSX composite index was down 78.59 points, or 0.35%, at 22,190.53.

Energy shares declined 2% as oil prices fell following a complex deal brokered by producer group OPEC+ to extend various layers of output cuts, most of them into 2025.

"TSX had a very nice May but still underperforming significantly versus the US markets year-to-date. It's just probably due to oil prices kind of being flat year-to-date, so not really helping there," said Barry Schwartz, chief investment officer at Baskin Financial Services.

Materials stocks, also under selling pressure, were down 0.1%, led by losses in metal miners such as Filo Corp , SilverCrest Metals and Ero Copper Corp , among others.

However, investors were squarely focused on the BoC's rate decision on Wednesday.

The central bank is expected to trim interest rates to 4.75%, according to three-quarters of economists in a Reuters poll that showed three further cuts this year, with the last one hanging on a knife's edge.

The loonie was marginally lower against the U.S. dollar at C$1.3609.

The expectations of a rate cut were boosted after the Canadian economy expanded at a slower-than-expected pace in the first quarter, data showed on Friday.

"It's a done deal, whether it's June or July, it's happening," Schwartz added.

Across the border, Wall Street opened higher, lifted by semiconductor and megacap growth stocks ahead of a data-packed week.

Among individual stocks, shares of GFL Environmental Inc jumped 6.8% to touch a near four-week high after a report by CFTN said the waste management firm was considering potential offers from private-equity firms.

(Reporting by Shubham Batra; Editing by Ravi Prakash Kumar and Shreya Biswas)