Royal London, owner of Bright Grey, said disruption caused by its takeover of Scottish Provident three years ago had hit service levels following criticism from advisers.
Some intermediaries described the service from Scottish Provident in particular as "abysmal" and "a disgrace" after Royal London last week reported lower first-half sales of protection products.
Royal London said Scottish Provident's recent history, where it passed through the hands of a number of owners including Abbey, Santander and Resolution, had impacted service levels.
It added there was less attention paid to investing in service in the run up to its acquisition of the business in 2008.
But it said it is committed to "improving all aspects" of Scottish Provident, pointing out it had changed its critical illness plan and lowered prices.
A spokesman said: "While these initiatives will not have had an impact on our result in the first half of the year, they have created an excellent platform for the second half.
"We have already seen the benefit of this in July with new business 10% higher than in July last year.
"We will continue to invest to give IFAs better service with the ambition of ensuring that Scottish Provident customers get the high level of service already available through Bright Grey."
Last week, Bright Grey and Scottish Provident reported a combined drop of 17% in sales of new protection business for the first six months of the year.
Advisers criticised the providers for service levels. IFA Harry Katz said service levels, from both Bright Grey and Scottish Provident, were "abysmal", while another unnamed advisers said Scottish Provident's administration and service levels are "a disgrace".
