Having led polls throughout 2017, the red-green alliance has fallen behind its rival conservative coalition, which forms the current government, for the first time in nine months, in a poll conducted by broadcaster NRK and Norstat.
The poll, conducted between the 8th and 14th of August, shows 27.1 percent of voters saying they support Labour.
Should the numbers hold on the September 11th election day, Conservative (Høyre) party leader Erna Solberg would win another four years as prime minister.
The result represents a drop of six points in support for Labour from NRK's June poll and the first poll to show a conservative bloc majority since December 2016 – even without the support of the Venstre (Liberal) party, a conservative coalition ally that is forecast to achieve under the four percent vote threshold required to enter Norway's parliament.
Labour Party leader Jonas Gahr Støre called the figures “bad and far too low”.
“[We] have not been clear enough about what this election is about,” he told NRK.
“It's my job to get people off the fence,” he added.
Solberg, whose party received a 0.1 point increase to 23.8 percent, said the result was related to what she considered negative campaigning by Labour.
“When Labour speaks systematically about how bad everything is in Norway, and how bad the government is, this is not the reality people recognise for themselves,” she told NRK.
Ketil-Solvik Olsen, deputy leader of the nationalist Progress Party, the junior partner in the coalition, was also satisfied with the result.
“This is a very good poll for us. It is inspiring, both for me and everyone else in Progress, who are working so hard on the campaign,” he said.
The poll shows a 2.7 percent increase in support for Olsen's party since June.
Olsen said that Labour had not been strong enough in opposition.
“This shows that they haven't done very well in opposition. Opposing everything for four years and campaigning on offering better solutions… that is not convincing,” he told NRK.