Some 12,000 pensioners were left without annuity payouts after a blunder by insurance giant Legal & General.
The company said it was working to sort out the problem but it was a "one-off" which would not affect the pensions of its other 650,000 annuity customers.
Those who failed to get paid may have to wait until Tuesday for their cash to clear the banking system, the company said, but it hoped most customers will get their cash this morning.
L&G urged people left short because of the computer glitch to contact the company and promised to ensure no one is left out of pocket.
A spokeswoman said L&G would recompense customers incurring any extra costs or bank charges.
She added: "This is a one-off event just affecting customer payments due on 21 February. No other annuity customer payments are affected.
"We regularly make payments to over 650,000 customers and have been doing so for over 35 years. This is the first time we've not been able to make payments on time." Affected L&G customers should call its help line on 01737 375151.