Mette Frederiksen, leader of the Social Democrats, confirmed the change in policy regarding her party's traditional ally on Monday.
Frederiksen told newspaper Jyllands-Posten that she would seek to form a minority government rather than form a coalition with any of the other parties on Denmark's left.
“We will look to form a minority government provided that the share of seats we gain at the next election allows it,” she said.
That would require parties in the so-called ‘red block' of the Danish parliament – those on the left or left of centre – to gain the majority of the seats.
The Social Democrats are the largest party in the Danish parliament, but the red block is currently in opposition with the ‘blue block' parties on the right holding the overall majority.
Frederiksen's decision breaks a 25-year old agreement between the two parties to go into government together.
But a single-party minority government would provide more room for manoeuvre in a time in which lines between the red and blue blocks have become more blurred, according to the Social Democrat leader, who specifically cited immigration as a key issue related to the break with the Social Liberals.
The Social Democrats have been criticised by other opposition parties for on several occasions following the line of the government and anti-immigration Danish People's Party on emotive issues including immigration law, the ban on burkas and a divisive plan to tackle social problems in underprivileged areas termed ‘ghettos'.
“I do not wish to be a the forefront of a Denmark that does not have immigration [udlændingepolitikken, ed.] under control,” Frederiksen said.
According to Jyllands-Posten's report, Frederiksen sees strict immigration rules and the introduction of further, even stricter laws on migration as more important than any other political issue.
Morten Østergaard, leader of the Social Liberal party, told the newspaper that his party would maintain its current position, including on immigration, where it wants a relaxation on some existing laws.
Frederiksen's announcement does not change the Social Liberal line, Østergaard said.
The Social Liberal leader also suggested the move by Frederiksen was the first step in a potential alliance between the Social Democrats and anti-immigration populists the Danish People's Party (DF).
"What's going on, Mette? Did you just grant [DF leader] Kristian Thulesen Dahl a free pass to set (Denmark's political) course?", Østergaard wrote in a tweet.
The widening gap between the two parties became increasingly clear last week during parliament's final debates before the summer recess.
During the sessions in parliament, the Social Liberals, along with the environmentalist Alternative party, broke with tradition by electing not to back a mutual ‘red block' statement put forth by Frederiksen's party, instead preparing their own – containing several explicit criticisms of Social Democrat policies.