Santander was the most complained-about mortgage firm in Q1 2017, according to new figures from the Financial Ombudsman Service.
The FOS says the lender got 736 new complaints made against it in the period.
The lender with the second-highest level of new complaints in the period was Lloyds, with 602, followed by Barclays (482), RBS (347) and Nationwide (299).
The mortgage firm with the most complaints upheld against it in Q1 2017 was Connells, owned by Skipton Building Society.
The FOS upheld 43 per cent of all mortgage grievances against Connells.
Across the whole mortgage market the FOS upheld an average of 24 per cent of claims.
The mortgage company with the second-highest level of complaints upheld was Countrywide Principal Services, with 37 per cent.
Following Countrywide was Kensington Mortgage Company (35 per cent upheld), The Mortgage Works (also 35 per cent) and Barclays (29 per cent).
Six months ago the firms with most complaints upheld against them were Santander and Connells. Both saw the FOS side with consumers in 36 per cent of cases.