Santander UK has blamed "ongoing competitive pressures" for a 21% fall in first quarter pre-tax profits to £414m, Sky News reported.
The Spanish-owned lender, which is the country's fifth-largest bank, said it had booked £60m in credit impairment losses between January and March compared with £13m in the same period last year.
It said the higher sum included further writedowns on its loans to failed construction-to-support services giant, Carillion.
Santander said the UK business, while growing customers, was suffering the effects of the slower economy and weak interest rate environment with fee income falling 2% on the first quarter 2017.
It reported consumer credit growth easing from double-digit rates in 2017 and said corporate loan growth had proved "volatile".
The bank said it would continue to keep an eye on costs over the remainder of the year.
Chief executive Nathan Bostock said: "Our first quarter results have been impacted by ongoing competitive pressures in the UK.
He added: "Cost discipline remains an area of particular focus for management, with targeted actions expected to reduce the cost run rate over the year and deliver operational efficiencies."
Santander is the first major UK bank to report its progress during 2018 so far - with results due from Lloyds, Barclays and RBS later this week.