UK Treasury bills are considered low risk because they're issued by the UK's Debt Management Office with the backing of the UK Government. Read more here.
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The UK government has a “AA”/AA/Aa3 credit rating from all major credit rating agencies, reflecting its strong financial position and ability to repay its debts.
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This means that investors in UK Treasury bills are very likely to receive the amount they invested, along with the agreed-upon yield, at maturity.