Lawmakers of Thailand’s main opposition party on Sunday resolved to immediately quit parliament in protest at the government of Prime Minister Yingluck Shinawatra, which they say has lost its legitimacy, the party’s leader said.

Abhisit Vejjajiva said lawmakers made the decision on Sunday after concluding that Yingluck’s government had lost its legitimacy when members of her Pheu Thai Party refused to accept a ruling by the Constitutional Court last month overturning a legislative attempt to change the way the Senate is elected.

“The government of Yingluck has dared to defy the ruling of the Constitutional Court,” the former prime minister and member of the Democrat Party, said, referring to the party’s initial reaction to the court’s November 20 ruling.

The argument is the same one made by Suthep Thaugsuban, a former Democrat executive who left parliament previously this year to lead a demonstration against Yingluck. Many see her as a front for her brother, fugitive former prime minister Thaksin Shinwatra.

Suthep will on Monday lead a 20-kilometre march on the seat of the administration in his “final battle” to topple the government.

Suthep has set Monday as the final battle in his campaign to oust Yingluck and end the political influence of Thaksin, who is seen as the de facto leader of the Pheu Thai Party.

Suthep has called for protesters to gather at Government House, the seat of the administration. The Democrats’ en masse resignation will allow them to join the protest as ordinary citizens.

Thaksin’s Pheu Thai party, which is strong in the north-east and north, won 265 of the 500 contested seats in July 2011 elections, with about 15 million votes, against the nearly 12 million garnered by the Democrats.

Many of the anti-government protesters in Bangkok come from the southern provinces, a traditional Democrat stronghold.

Yingluck on Sunday, in a televised message, said she was ready to dissolve parliament and call for a new election within 60 days, if that is what the people want,” The Nation online news reported.

The protesters have repeatedly said a legislative dissolution is not enough, since they know a new election would bring back another Thaksin-led government.

Suthep wants an appointed prime minister who will set up a People’s Assembly to reform the political system, with an aim of preventing Thaksin’s return to politics.

The Bangkok protests have been ongoing since November 1, when the lower house of parliament pushed through an amnesty that would have pardoned Thaksin from a two-year prison sentence for abuse of power.

Although the bill was later rejected by the Senate, its introduction has been enough to polarise society. While Democrats were opposed to the attempt to pardon Thaksin, many others worried that it went too far in providing amnesties for scores of others linked to past violence, both for and against Thaksin.

Suthep has led a campaign to paralyse the government since November 24.

He claims Yingluck’s government lost all legitimacy when her party openly rejected the Constitutional Court’s November 20 ruling on Senate elections. The Senate is currently half elected and half appointed. The government’s measure would have made it fully elected.

The protests have already claimed four dead and more than 265 injured.