He was initially set to return to his post by the end of the year, but his leave was extended until the end of January. On Tuesday morning, Sweden's Expressen newspaper broke the story that he would remain on sick leave.
A spokesperson for the Sweden Democrats had previously announced that Jimmie Åkesson would “provide information” about his health on Tuesday morning.
Mattias Karlsson has been the party’s acting leader in Åkesson’s absence and some have suggested that he could take over permanently.
Last week, Sweden's Expressen newspaper published parts of an email written by Jimmie Åkesson in which he said that he had full confidence in Karlsson.
The Sweden Democrats are the third largest political party in Sweden, scoring a record 12.9 percent of the vote in September’s election.
On Monday the party announced that it had chosen a new party secretary, with Richard Jomshof set to replace Björn Söder, one of the anti-immigration group's most senior and controversial officials.
Most recently, Söder questioned how "peaceful" Islam is, following the Paris terror shootings.
Richard Jomshof, a 43-year-old who is a former teacher, has been a member of parliament for the Sweden Democrats since 2010 and is based in Helsingborg in southern Sweden.