Shares of HDFC Bank fell 3 per cent on Wednesday, dragging the benchmark Nifty 50 index after the country's top private lender said its newly completed merger with HDFC Ltd would hit some key financial metrics.

The bank has said it expects some "pressure" on its net interest margins (NIM) and forecast a rise in non-performing assets (NPA), according to analyst reports following a meet with HDFC Bank's management held early in the week.

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HDFC Bank is expected to report its first consolidated earnings next month, though a date has not been announced yet. The two companies merged effective July 1, creating a $40 billion behemoth.

About 11.6 million shares changed hands in early trade against a 30-day average of 21.1 million.

HDFC Bank shares are down 3 per cent so far this year, underperforming 6 per cent gains in the bank index and 11 per cent gains in the Nifty index.