Obscure rules from the Bank of England could lead to trouble for credit unionsGETTY IMAGES

Wealthy investors are being urged to think twice before ploughing their savings into credit unions, because obscure Bank of England rules mean it could land the institutions in financial trouble.

Rich investors have been pumping money into credit unions, which are non-profit organisations owned and run by their members, amid a rise in the popularity of “social-impact investing” — investments that make a difference to society and generate a financial return. Credit unions tend to lend money at more reasonable rates of interest than mainstream lenders to people who would otherwise have difficulty getting a loan.

Total deposits have risen from £450 million to £1.3 billion in the decade to September 2017, with today’s accounts typically offering savers rates of between 0.5 and 1 per…