President Sirisena sacked Prime Minister Wickremesinghe on October 26 and replaced him with Mahinda Rajapaksa, a controversial former president, and suspended parliamentary proceedings until November 16.
Critics of Sirisena's actions say Parliament was suspended to give Rajapaksa time to gather enough support to survive a no-confidence vote when lawmakers reconvene November 14.
The United States and the European Union have urged the president to immediately summon parliament and let deputies decide who is to lead the country.
"I have to agree with the majority of parliament who believe that the president's actions are undemocratic, unconstitutional and against all norms of parliamentary procedure", added Jayasuriya, who is originally from Wickremesinghe's United National Party but holds a neutral position in parliament.
The sudden development came amid growing tensions between Sirisena and Wickremesinghe on several policy matters and the President has been critical of the Prime Minister and his policies, especially on economy and security.
Rajapaksa is known as a Sri Lankan nationalist, while being close to China.
"Until the new faction (of Rajapakse) is able to prove a majority in parliament, I will recognise the situation that prevailed prior to October 26", Jayasuriya said in a one-page statement.
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Almost all politicians who left the UNP took up ministerial posts in the new cabinet. President Sirisena had recalled for parliament for November 14.
Jayasuriya said it would be "inappropriate" for him to side either way - but official sources said he took the unusual step to avoid publicly clashing with Sirisena.
Some members of his party have alleged they were offered portfolios and huge amounts of money to switch allegiances.
Before Wickremesinghe's dismissal, the UNP held 107 seats.
The deposed leader has demanded a vote to prove his majority.
Both rivals have been marshaling numbers behind the scenes, worrying civil society groups who protested Sunday in the capital Colombo against the horse-trading.
Meanwhile, Rajapaksa supporters have planned a rally near the nation's parliament on Monday.
Sirisena made the decision after his United People's Freedom Alliance pulled out of the national unity government which it had formed with Wickremesinghe's United National Party.
He resigned as the deputy minister of Labour and Foreign Employment to leave the Sirisena camp and join Wickremesinghe.
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