Toby Strauss has signaled that the company has "struggled" to manage in the wake of the major changes announced in the Budget.
He suggested the Government needs to give firms more wriggle room to allow the reforms to bed in.
In a letter to the Telegraph, he said: "In the absence of any breathing space, we have concerns that the industry is in danger of reaching – and in fact breaching – its capacity to cope.
"Many of our own processes have struggled in the wake of the ongoing changes and as a result our service levels are in some areas falling short of the high standards our customers expect of us.

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"I fully appreciate the impact that our service issues have had, and are having, on our customers and we are working hard and investing significant resources to ensure that we fix this as quickly as possible."

Scottish Widows is investing tens of millions of pounds to help it deal with the changing landscape, he said.

He said: "This involves new customer service staff, upgrading IT systems and investing in training programmes.
"We are working with financial advisers, employers and the payroll industry, where we are also seeing capacity challenges.
"The most recent Budget changes will enable consumers not only to have greater flexibility with their savings but also to have an ongoing dialogue with their adviser. In the future, retirement won't be a one-off conversation.
"So to deliver real benefit to consumers, these changes must be allowed to bed in before any further change is considered, so that we can ensure we have robust approaches in place to help our customers with their financial decisions."
He added the last three years in the pensions world had been "unprecedented" but believes the changes have created "fairer and more transparent" conditions for consumers overall.