The development follows a weekend of frantic talks and deal-doing.
HSBC bought the UK arm of stricken Silicon Valley Bank for a symbolic one pound on Monday, rescuing a key lender for technology start-ups in Britain and helping curb the fallout from the biggest bank collapse since the financial crash.
The move comes after U.S. authorities moved to shore up deposits and stem any wider contagion from the sudden collapse of its parent Silicon Valley Bank.
The deal, which sees one of the world's biggest banks, with $2.9 trillion of assets, take the doomed British arm of the tech lender under its wing, brings to an end frantic weekend talks between the government, regulators, and prospective buyers.
"HSBC is Europe's largest bank, and SVB UK customers should feel reassured by the strength, safety and security that brings them," Britain's finance minister Jeremy Hunt said.
"We were faced with a situation where we could have seen some of our most important companies - our most strategic companies - wiped out, and that would have been extremely dangerous," Hunt told reporters.
Asked about HSBC's white-knight role, Hunt said the finance ministry's priority had been to avoid using British taxpayers' money. One pound is worth $1.21.
The Bank of England said it had organised the sale to underpin confidence in the financial system and minimise any fallout for British technology firms.
It said deposits at the bank were safe as a result of the sale, and that the wider banking system was safe.
Britain's blue chip FTSE 100 index of shares was down 1% in early trading, after sliding 1.7% on Friday amid wider global market turmoil sparked by SVB's collapse. HSBC shares were down 1.7%.
"On the face of it appears a good deal," Richard Marwood, senior fund manager and HSBC investor at Royal London Asset Management, said. "SVB lacked liquidity and depositor confidence - HSBC has both of those in spades."
SVB UK is ringfenced from the U.S. group, and HSBC said the assets and liabilities of the parent company were excluded from the transaction.
"This acquisition makes excellent strategic sense for our business in the UK," HSBC CEO Noel Quinn said in a statement.
SVB UK has loans of around 5.5 billion pounds and deposits of around 6.7 billion pounds, HSBC said, adding the takeover completes immediately.
The Bank of England said SVB UK had a total balance sheet size of around 8.8 billion pounds.
Unlike the United States, Britain has not announced broader liquidity measures for the banking system.
Dozens of listed British companies issued statements on Monday about their exposure to SVB UK, seeking to reassure investors, or in some cases warn them, just as news of the rescue deal was announced.
THG, an online retail platform, card maker Moonpig and Naked Wines issued statements saying they were either not exposed or did not expect to be affected. Diaceutics had warned its liquidity would be impacted.
Others had also examined buying the bank. Bank of London said on Sunday it had submitted a formal proposal. SoftBank-owned lender OakNorth Bank also weighed a bid, a person with knowledge of the talks told Reuters.
Abu Dhabi state-backed investment vehicle ADQ was also looking, according to media reports.
(Additional reporting by Sinead Cruise, Sarah Young and Kate Holton in London and Yadarisa Shabong in Bengaluru; Editing by John O'Donnell, Jason Neely and Catherine Evans)
HSBC Rescues British Arm Of Stricken Silicon Valley Bank