In his second and final interview since announcing on Thursday he would step down, Speirs told FIVEaa on Friday afternoon that there were “two or three people, max” in the Liberal Party room who “decided to publicly undermine my leadership, to call it into question, rather than come to me with concerns”.

“I’m really disappointed on behalf of my other colleagues because, really, I led a party that was uniquely united, but it doesn’t come across that way if there’s just one or two people providing information to the media,” he said.

“I’m not going to name them… but if I got the impression that those people were rewarded as a consequence of their bad behaviour, I would find it very difficult to stay quiet and I would find it very difficult to continue in the Liberal Party, quite frankly.”

The comments come ahead of a joint-party room meeting on Monday to elect Speirs’s successor, with his deputy, John Gardner, and moderate colleagues, Josh Teague and Vincent Tarzia, the most likely candidates.

Asked if he would go to the crossbench if the MPs he accused of undermining him were elected to leadership positions, Speirs said: “I would find it difficult to remain in the Liberal Party.”

“I am not making that as an idle threat, but… my colleagues know who these people are, and when they cast their vote on Monday for the new leader, they’ll be casting a vote for continued unity or for a very, very difficult time.

“I’m not threatening to go to the crossbench today, but I will watch very carefully to make sure that bad behaviour isn’t rewarded, and that game playing isn’t rewarded, because South Australians deserve better.”

Speirs later added that he was “not going to be forced out of my job by a small minority and allow those people to be patted on the back and go on to greater things”.

“The party has to grow up if it wants to govern in the future,” he said.

The leadership contest to replace Speirs, who is avowedly factionally unaligned, will go to a ballot unless a deal is worked out between the three likely candidates over the weekend.

Speirs said he would not attend the joint party room meeting on Monday but would cast a proxy vote. He did not publicly endorse a candidate.

The former Environment Minister in the Marshall Government, who was elected to the party leadership in April 2022, said he is now taking “a few weeks off” to spend time with friends and family and travel to his cousin’s wedding in Scotland.

Speirs’s resignation followed months of speculation about his leadership, which intensified after the party’s damaging loss in the Dunstan by-election in March.

He said he made the decision to resign “probably a couple of weeks ago” and revealed he also considered resigning after the Dunstan result.

“I thought, should I go immediately after that? And then I made the decision not to do that. I thought it would leave the party in the lurch,” he said.

“But a couple of weeks ago, when the speculation about my leadership started to appear in the media – I find that quite disturbing.

“At the end of the day, a very small group of people can be hugely destabilising, and I didn’t want to be the sort of leader who limped along.”

Speirs has committed to staying as the local MP for the southern suburbs seat of Black until at least 2026.

He has also not ruled out a return to the Liberal frontbench, saying: “I could happily serve in a shadow ministry at some point in the future”.

“I’m not shutting the door on anything.”