"It's been hard for us to foresee this increase," Ulla Östman Krantz, an analyst at Sweden's National Social Insurance Agency (Försäkringskassan) told the Dagens Nyheter (DN) newspaper.
"It's a serious matter that sickness benefits claims are on the rise again and we don't really know what lies behind it."
The agency had been allotted 19 billion kronor in 2012 to cover the costs of sickness benefits claims (sjukpenning), but according to an updated forecast carried out in October, the agency is set to face claims for the year totalling 21.5 billion kronor.
In Sweden, employers are expected to provide sick pay at 80 percent of workers' salaries for the first two weeks of illness, after an initial qualifying day (karensdag) which is unpaid.
If workers are home due to illness for more than two weeks, the social insurance agency then starts paying the benefits, up to a maximum of 364 days within a 15-month period.
Following the October forecast, the agency, which is charged with delivering a variety of payments associated with Sweden's elaborate social safety net, has gone to the government to ask for permission to overspend its budget by 2.5 billion kronor.
"We have to continue paying out sickness benefits, even if we really don't have the money," said Östman Krantz.
The Social Affairs Ministry has indicated it plans to consider the request quickly and ensure that agency can cover the benefits requests.
There are also plans to increase the agency's budget for 2013.