Last month, President Sirisena sacked Ranil Wickremesinghe as prime minister and named former president Mahinda Rajapaksa as the new prime minister and suspending parliament.
Sirisena signed an official notification dismissing the 225-member assembly with effect from midnight, clearing the way for a snap election almost two years ahead of schedule.
A minister, speaking on condition of anonymity, said the official notification would take effect at midnight on Friday.
The notice will become official once it's published, and it is required to include dates for nominations for fresh elections.
The dissolution is the latest twist in a constitutional crisis that began October 26 when Sirisena unexpectedly fired Ranil Wickremesinghe, who served since 2015 as prime minister in a unity government with Sirisena.
President Sirisena has suspended parliament until next Wednesday as he and Rajapakse try to gather enough support among lawmakers to vote Wickremesinghe out, an effort that looks to be failing.
As protests rocked the streets of the capital Colombo, however, the United National Party voiced its indignation on social media, saying it "is illegal and goes against the constitution".
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"He has robbed the people of their rights and the democracy that we have enjoyed", the UNP said.
"Unfortunately, we fear that recent actions, if not corrected, will threaten your country's democratic development and derail the progress made in recent years", said the letter to Sirisena. Previously, Sirisena's party admitted to not having the votes to confirm ex-strongman Mahinda Rajapaksa as the new prime minister.
Amid rumours that Sirisena may seek to delay matters further, the European Union, in a joint statement with Norway and Switzerland, said that parliament should vote "immediately when reconvened".
"At the moment we have 104 or 105 MPs", UPFA's spokesman Keheliya Rambukwella told reporters, adding that the Sirisena-Rajapakse group hoped to secure support from "crossover" legislators.
The admission, which came despite Sirisena's earlier claim that he had the support of 113 legislators when he sacked Wickremesinghe, had fuelled speculation that he would go for snap elections. Several legislators have said they were offered millions of dollars to switch allegiance and at least eight have already jumped to the president's side.
The next elections weren't due until 2020, and under the constitution, the president can dissolve parliament only after completion of 4 1/2 years of its term.
On Thursday Wickremesinghe thanked his supporters and urged them not to give up in the showdown.