Unum is the life & protection insurer with the highest rate of upheld complaints to the Financial Ombudsman Service (FOS).
The figure comes from FOS' report of complaints made to it during the second half of 2014.
Unum had 51 new cases brought before the Ombudsman while 75% of complaints were upheld against the insurer.
The Ombudsman noted that the number of complaints received was likely to correlate with the size of the business.
Cardif Pinnacle had 312 new complaints against it - the highest number of new cases among companies offering protection-type products. The number of complaints resolved in favour of the consumer was 67% - the second highest upheld rate.
AXA PPP had the third highest number of complaints ruled in favour of the consumer, with 43% of complaints upheld. The Ombudsman had received 84 new complaints against AXA PPP.
The majority of complaints received by FOS overall concerned PPI which accounted for 65% of the 161,649 complaints received by FOS.
Non-PPI cases remained at a similar level to the first half of 2014 while the rate of complaints upheld in the consumer's favour was 52%, ranging from 4-98% across different businesses.
In reporting the figures FOS combined critical illness; income protection; long-term care; PMI and dental insurance; and term assurance within its "general insurance/pure protection" category.
In a statement Unum said: "Unum takes every complaint seriously and examines each one in order to learn and improve, working closely with the FOS. Over the last few years, we have made numerous enhancements to our claims process which is fair and robust.
"We must emphasise that the number of cases brought before the FOS in H2 2014 was only approximately 3% of all Unum's claims in that period. The average number of our cases going to the FOS also halved in 2014.