Akin -- the Republican nominee for U.S. Senate -- notified the House Ethics Committee this week his congressional reports didn't include income from his state legislative pension, the St. Louis Post-Dispatch reported.
"This was an unintentional oversight and I regret any inconvenience this may cause," Akin told the House committee.
The Post-Dispatch said Akin's office acknowledged the underreporting Friday, following the newspaper's inquiry into the absence of Missouri legislative pension payments on his congressional disclosure form.
Akin last year amended personal financial disclosure reports to reflect property assets worth at least $355,000, following the newspaper's investigation, the Post-Dispatch said.
The six-term St. Louis-area congressman is challenging Democratic incumbent Claire McCaskill in a campaign that is being closely watched for its potential affect on which party will control the Senate after the November election. It also grabbed attention when Akin made controversial statements about rape.
Two polls released Wednesday -- by Rasmussen Reports and Public Policy Polling -- indicated McCaskill leading Akin by 6 percentage points.
The Rasmussen poll has a 4.5-percentage-point margin of error and the PPP poll has a 3.7-percentage-point error margin.