Mr Manele has pledged not to stand down ahead of the May 6 vote, despite the defection of 10 government MPs including former ally Manasseh Sogavare.
The first-year prime minister lost his majority in parliament this week after the mass walkout, reportedly led by Mr Sogavare, the three-time former prime minister and Mr Manele's finance minister.
The grouping has not given reasons for the move, though local outlet In-depth Solomons has linked it to internal dissatisfaction with deputy prime minister Bradley Tovosia's conduct.
Mr Tovosia also resigned on Monday, a move seen as an attempt to calm waters and restore support for Mr Manele's three-party coalition.
It appears not to have worked.
On Tuesday, the rebel government MPs aligned with Opposition Leader Matthew Wale for a photo together before a sitting of parliament, showing their new grouping and their strength in numbers: 25 of the 50-seat parliament.
Claiming the support of an absent 26th MP, the group would boast a majority and could pass next week's no-confidence motion, moved by Honiara-based MP Gordon Darcy Lilo.
Mr Manele successfully moved to adjourn parliament until next Tuesday, though not before Mr Wale invited him to fall on his sword.
"It will be also expedient if the prime minister could consider stepping down immediately and not allowing us to go through the uncertainty to Tuesday the sixth," he said.
"That's a matter for him to prayerfully consider."
Mr Manele declined the invitation.
"I will not step down before that date. I will allow that motion to be moved on the sixth of May and then debate it and we'll see how it goes," he said.
Both men called for calm in the community, and shared a handshake at the end of proceedings.
"I urge the people of the Solomon Islands to remain calm and respect the democratic process," Mr Manele said.
The dramatic ructions show a split in the coalition and its parties, particularly the OUR Party, which counted both Mr Sogavare and Mr Manele as members.
Mr Manele was foreign minister in Mr Sogavare's last government, and emerged as a compromise candidate for prime minister in government formation talks last May.
After losing six members of cabinet on Monday, Mr Manele looked a forlorn figure in parliament on Tuesday, though the unpredictability of Solomon Islands politics means it is impossible to count him out.
It is also unclear who might take the prime ministership in a secret ballot which would be held in the aftermath of a successful no-confidence motion.Â
Mr Manele won the May 2024 vote by 31 votes to 18 over Mr Wale.
Political instability is nothing new in Solomon Islands: there have been 11 changes of prime minister this century.
The Melanesian nation of 750,000 people also faces enormous challenges, not least economically: it is the poorest nation in the Pacific per capita.
Mr Lilo's motion is his second attempt to topple the government six months.
Last December, he tabled a similar motion - citing concerns with cost of living pressures and government corruption - only to withdraw the motion before it went to a vote.
"The situation that we are in right now, I think it requires good encouragement," he said in parliament.
"Stick together. Be strong. Run a country in a responsible way ... (don't) derail confidence in running the government of our country."
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