Santander said it would offer free overdraft extensions and mortgage payment holidays to customers who have been affected by the flooding.
Homeowners' circumstances would be reviewed on a case-by-case basis and may include conversion of mortgage loans to interest-only payments.
Farmers would be offered extensions to current facilities for up to a year and businesses would get at least six months extensions.
Customer overdraft extensions for business and personal accounts would be made without renewal or extension fees being levied.
RBS said it would grant customers a payment holiday and comes on top of its January announcement of a £250m storm business fund.
It said: "Today the bank has announced that RBS and NatWest mortgage customers with flooded homes will be able to apply for a three month mortgage repayment holiday.
"In addition, the bank will send specialist business support teams to affected areas in the coming weeks to help (companies) with short-term financial problems as they conduct urgent repair work and deal with loss of trading income."
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