The correction of the error puts the left and the right-wing blocs slightly further apart in a still very tight race, with 144-142 seats in preliminary counting, rather than 144-143 as officials initially stated after the election.
The left-wing bloc is made up of the currently governing Social Democrats and Greens, with the informal support of the Left Party in parliament, and the right-wing of the so-called 'Alliance' of the Moderates, Centre, Liberals and Christian Democrats. The far-right Sweden Democrats are not part of either bloc.
Swedish news agency TT that first noticed that the preliminary election results had suddenly changed on the Election Authority's website, with the Centre Party and Sweden Democrats now at 30 versus 63 seats.
“A district in northern Västra Götaland had got the elections mixed up. They had reported the figures for the county election instead of the parliamentary election,” Lars Aden Lisinski, a press spokesperson for the Election Authority, told the news agency after investigating.
- What's next for Sweden after election nailbiter?
- Sweden faces uncertainty after dead-heat election
- Everything you need to know about the Swedish election
Sweden holds three elections on the same day: national (riksdag) regional (landsting) and local (kommun). Votes are counted manually at each polling station on the night of the election and the counting process is open to the public to come and watch. The number of votes for each party are then reported to the county administrative authorities as 'R' (riksdag), 'L' (landsting) and 'K' (kommun), and then get counted again.
But one of the polling officers in the Västra Götaland region accidentally reported the regional votes for the landsting as 'R', which meant they first got added to the national number of votes for parliament.
“In Västra Götaland we don't use the term 'landsting' but we call ourselves a 'region'. It was late at night and I got it wrong,” the polling officer, who discovered and corrected the mistake on Monday, told Aftonbladet.
Overseas and late votes, which are counted on Wednesday, can still swing the results for Sweden. The two blocs are currently less than 30,000 votes apart. In the election of 2014 around 250,000 votes were added in late counting, around 200,000 ballots that had not been counted on the night and 50,000 overseas votes.
Past experience suggests that overseas voters tend to favour conservative parties, with 28,000 Swedes abroad voting for the right-wing bloc in the 2014 election, compared to 17,000 who voted for the left-wing bloc. However, there is no predicting how the remaining late votes are going to fall.