By Selena Li and Lawrence White

HONG KONG/LONDON, Oct 27 (Reuters) - HSBC's shares slid 6% on Tuesday after revenue undershot estimates and growth at its wealth business slowed, highlighting the challenge big banks face to sustain recent record profits.

Europe's largest bank reported a 32% surge in pretax profit to $9.6 billion for the July-September quarter, in line with estimates. But investors and analysts focused on the lack of upgrades to the bank's performance guidance and underwhelming third-quarter revenue of $18.6 billion against forecasts for $19 billion.

The drop in the bank's shares underscores how recent strong results from European lenders have increased investor expectations for performance, even as the impact of the Iran war on inflation and interest rates has created a tough environment for banks. The shares remain up 9% for the year to date.

HSBC also launched a share buyback of up to $1 billion, the same amount as the previous quarter and a move that had boosted shares in previous reporting periods but is now largely expected by investors.

"There is sometimes a market expectation that we upgrade guidance or targets," CFO Pam Kaur told reporters on a conference call when asked about the share drop.

Analysts noted that the bank's net interest income slightly missed expectations, while fee income from its key wealth growth pillar was steady but unspectacular.

Escalating deposit competition in Hong Kong and the UK, HSBC's two core markets, are expected to push up interest expenses, they said. Time deposits in the bank's Hong Kong portfolio rose two percentage points quarter-on-quarter to 38%, but Jefferies analysts noted that the shift to chase higher deposits by offering higher interest rates is expected to erode margins.

Revenue for HSBC's wealth division was flat at $2.7 billion. That compared with 15% and 23% year-on-year revenue growth in the first two quarters.

The performance of HSBC's wealth business is under particular scrutiny because the British bank, like its peers, is focusing growth on fee-generating businesses such as wealth management as lending comes under pressure from falling interest rates.

Analysts warned that Asia-focused banks are bracing for a slowdown in wealth momentum following robust gains driven by Chinese capital inflows.

Kaur said on the call that China's tightening of wealth flows has prompted clients to shift from other banks in Hong Kong to HSBC.

"We picked up more business."

BRITAIN'S BANKS POISED FOR TAX HIKES

HSBC benefited from the absence of legal provisions booked in the same quarter a year earlier. It also said expected credit losses tumbled 17% to $800 million, thanks to a reduction in charges linked to Hong Kong's real estate sector as well as its UK and Mexican wealth businesses.

British banks' strong recent results have prompted calls from trade unions and activist groups for tax hikes on the sector in finance minister John Healey's first budget, due on Wednesday.

Bank bosses have lobbied hard to avoid such an increase in sector levies, arguing it would curb lending and make Britain less competitive.

HSBC announced an interim dividend of $0.1 per share, the same as the previous quarter.

(Reporting by Selena Li in Hong Kong and Lawrence White in London; Editing by Edwina Gibbs and Susan Fenton)

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