Sussan Ley’s deal to reunite with the Nationals has damaged her credibility and potentially shifted some MPs into her leadership rival Angus Taylor’s corner, according to colleagues who are bracing for a spill as soon as this week.

As Ley told a press conference the majority of colleagues believed the two parties were “stronger together”, one Liberal MP privately feared the reunion would ultimately lead to the “destruction of the Liberal party”.

“It is in essence an abandonment of all Liberals that live in our cities. Why would they vote for a party that is in coalition with the Nationals, when the Nationals’ only policy objective is to be more rightwing than One Nation?” the MP, speaking on the condition of anonymity, said.

The Liberal and Nationals leaders announced a deal to reunite the two parties on Sunday afternoon, just 17 days after David Littleproud blew up the Coalition and declared the political alliance “untenable” under Ley in a rift over Labor’s hate speech laws.

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“It’s been a difficult time for millions of our Coalition supporters and many other Australians who rely on our two great parties to provide scrutiny and national leadership, but the Coalition is back together and looking to the future, not the past,” Ley said.

Under the peace deal, all of the former Nationals frontbenchers will be suspended from the shadow ministry until 1 March before resuming in their old portfolios.

Littleproud and the Nationals’ deputy leader, Kevin Hogan, will attend shadow cabinet and other senior leadership meetings during the period, even though neither will technically hold frontbench positions.

Both leaders agreed to major concessions to land the deal ahead of Ley’s self-imposed Monday deadline to reunite the parties before she pushed ahead with a Liberal-only frontbench.

Ley gave ground on her initial demand for six-month suspensions for the three senators who crossed the floor, while the Nationals agreed to accept some form of punishment, having previously insisted none was warranted.

The prospect of reuniting the Coalition divided the Liberals and put Ley’s fragile leadership under further strain.

Taylor, his fellow rightwinger James Paterson and senior shadow ministers Dan Tehan and James McGrath were pushing for the Coalition to quickly reform, as was the former prime minister John Howard.

But others – including many moderates – were comfortable with a longer-term split from the Nationals, particularly as the country party pursues more rightwing policies to combat the threat of Pauline Hanson and Barnaby Joyce’s One Nation.

Guardian Australia has spoken to five Liberal sources who believe the concessions to reunite the Coalition have damaged Ley’s standing and could shift a crucial bloc of undecided MPs into Taylor’s corner – even though the shadow defence spokesperson had pushed for the same outcome.

Ley dismissed the prospect of one-time supporters shifting their allegiance to Taylor.

“The overwhelming majority of my party room knows that the Coalition is stronger together,” she said.

Some Liberals are refusing to forgive the Nationals for breaking ranks to oppose the hate speech laws, which triggered a backlash of criticism from the party’s conservative members who wanted them to do the same.

One MP described the Nationals’ behaviour as “unconscionable” ahead of the 17-day split – and even worse during it.

“By joining the Nationals we are condemning ourselves to being seen as the junior partner to One Nation,” they said. “One Nation wins. We now have their wannabe cousins, the Nationals, dictating how Liberals should set their policy agenda.”

A separate MP, who previously doubted a leadership challenge would materialise but now considers a spill inevitable, said the decision to accept the shorter suspensions was a significant backdown that would tip the number’s in Taylor’s favour.

A leadership spill is considered highly unlikely at Tuesday’s Liberal party room meeting because of Senate estimates.

However, MPs are preparing for the possibility of a second party room being called for Friday.

Taylor would first need to resign from shadow cabinet to support a spill motion and contest the ballot.

Ley defeated Taylor 29 votes to 25 in the post-election leadership ballot with the support of a coalition of moderate, centre-right and unaligned MPs.

One moderate powerbroker said while Ley still retained their support, MPs were deeply disappointed at having conceded to the Nationals, particularly after the faction lost to fight to preserve a net zero target.

The former moderate Liberal MP Jason Falinski – who had supported a longer-term separation – said the compromise deal was “not a bad outcome”.

“To be honest, I thought the obstacles to the Coalition getting back together again were pretty high, and would have been difficult to surmount. So credit where credit’s due,” he told the ABC.

A senior conservative said Ley made the “sensible” decision to accept the deal, avoiding a repeat of the “circus” that played out in parliament last week when the two parties sat apart.