International Business Machines Corp. informed its employees this week that it will scale back the frequency of company contributions to its 401(k) retirement plan in 2013.

Beginning Jan. 1, IBM will make a single annual contribution to employee retirement plans at the end of every calendar year, according to an email sent to plan participants on Thursday from the company's senior vice president of human resources, Randy MacDonald. The Armonk, N.Y.-based technology giant had been making contributions twice a month.

Active full-time and part-time workers participating in the plan must be employed with IBM on Dec. 15 of each year in order to receive the company's matching and automatic contributions, according to the email. Company contributions will be deposited in employees' retirement accounts on Dec. 31, or on the last business day in December.

In his email, Mr. MacDonald specified that IBM is not planning to change its automatic and matching contribution rates, which are set at 1%-4% and up to 6% of eligible income, respectively.

“This change reflects our continuing commitment to invest in our employee 401k plans while maintaining business competitiveness in a challenging economic environment,” a spokesman for IBM said in an email. “IBM’s 401(k) Plans remain among the best in the industry — and the country — including our dollar for dollar match at 5% and 6% and automatic contributions of 1%, 2%, and 4% depending on the employee’s start date and years of service.”

Mr. MacDonald's email was posted online by the Binghamton, N.Y.-based Alliance@IBM, an advocacy group for IBM employees that has actively pursued a collective bargaining with the company.

The planned change to its 401(k) plan drew sharp criticism from the group's national coordinator, Lee Conrad.

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“We're asking IBM to reverse this decision,” Mr. Conrad said in an interview. “Most employees are very angry about this. What this means is that you could be working for IBM all the way up to December, and then suddenly not be eligible for the contributions from the company.”

Mr. Conrad also pointed out that the shift to an annual contribution schedule would mean that the company would become the primary beneficiary of interest accrued on funds set aside for the contributions. Under the old schedule, he said, that accrued interest belonged primarily to employees.

“People feel like their pockets are being picked,” Mr. Conrad said.

IBM employs more than 440,000 workers worldwide. Costs associated with the company's 401(k) plan for U.S. employees totaled $875 million in 2011, according to its last annual report to investors.