* TSX down 42.13 points, or 0.27 percent, to 15,591.08
* Five of the TSX's 10 main groups were down
TORONTO, May 11 (Reuters) - Canada's main stock index fell on Thursday, as a Moody’s downgrade of major Canadian banks struck financial stocks, and frequent-flyer points operator Aimia Inc plunged on news its program would be dropped by the country’s largest airline.
Financial stocks fell 0.8 percent, at one point trading at their lowest level in five months, after Moody's Investor Service cut the long-term ratings for Canada's six biggest banks, citing a more challenging operating environment.
Canadian Imperial Bank of Commerce fell 1.4 percent to C$107.25, while National Bank of Canada declined 1.3 percent to C$53.44.
Canada's largest pipeline company, Enbridge Inc, was by far the most influential mover on the downside, however, sliding 2.1 percent to C$54.75 after it reported a lower-than-expected quarterly profit. The stock tempered gains by the overall energy group, which climbed 0.3 percent.
At 10:21 a.m. ET (1421 GMT), the Toronto Stock Exchange's S&P/TSX composite index fell 42.13 points, or 0.27 percent, to 15,591.08.
Half of the index's 10 main sectors lost ground.
Shares in Air Canada, the country's dominant carrier, jumped 6.4 percent to C$15.84 after announcing it would start its own frequent-flyer program to replace Aeroplan, operated by Aimia.
Aimia stock sunk 58 percent to C$3.75 on the news.
The consumer discretionary group, which Aimia falls under, fell 0.2 percent, but the sector's losses were offset by a 4.8 percent jump to C$61.17 by Magna International Inc, after the autoparts maker posted a quarterly profit that beat estimates.
The materials group, which includes mining and lumber companies, added 1.1 percent.
Declining issues outnumbered advancing ones on the TSX by 127 to 117.
The index was posting 13 new 52-week highs and one new 52-week low. (Reporting by Solarina Ho; Editing by W Simon)