Complaints about home finance deals within the private banking arm, were nearly 80 for every 1,000 policies, Financial Conduct Authority (FCA) data showed.
Barclays said its private bank had 24 complaints against 302 accounts, which is how the high figure was generated.
Only UK Asset Resolution, which manages the failed bank assets from the financial crisis, attracted more complaints in the sector during the same period.
Barclays UK bank attracted 11 complaints for every 1,000 policies.
Mortgage complaints down
Overall home finance complaints fell during 2019 H2 from 8.7 to 8.4 complaints per 1,000 policies.
After Barclays, Blemain Finance Limited, part of Together, and Al Rayan Bank were the most complained about home finance providers with 49 and 39 complaints per 1,000 policies respectively.
Mortgages made up just two per cent of all complaints about regulated firms, with around half being upheld.
Complaints about all regulated firms jumped to 6.02m from 4.29m from the six months previous.
The increase was mostly driven by a 75 per cent increase in the volume of payment protection insurance (PPI) complaints received.
The product accounted for 62 per cent of all complaints received in the period, as the deadline for submitting queries drew to a close on 29 August 2019.
There was also a six per cent increase in all other complaint volumes compared with 2019 H1, from 2.18m to 2.31m.
After PPI, the most complained about products were current accounts, making up 10 per cent of all complaints, credit cards and other general insurance products.
Excluding PPI, the average redress per complaint upheld fell from £200 in the first half of 2019 to £184 in the second half.