A South Australian Liberal MP has been branded a traitor for defecting to the state Labor government and taking up a plum ministry as an independent.

Martin Hamilton-Smith made the shock announcement on Tuesday, saying he was putting the people of SA ahead of politics.

Opposition Leader Steven Marshall said Mr Hamilton-Smith’s defection was “an act of political betrayal and treachery”, coming so soon after the state election returned Labor for a fourth term.

“Mr Hamilton-Smith’s disgraceful decision is unrivalled in its treachery and duplicity,” Mr Marshall said in a statement.

Mr Hamilton-Smith had betrayed his electorate of Waite and the 456,000 voters who put the SA Liberals first at the March poll, he said.

“Mr Hamilton-Smith might think he can call himself an independent Liberal, but he is now a member of a Labor cabinet.”

Premier Jay Weatherill said Mr Hamilton-Smith would take on the portfolios of investment and trade, defence industries and veterans affairs.

The MP, a former parliamentary Liberal leader, said Mr Weatherill had reached out to him after the state election and he had considered his decision very carefully.

The pair met on Monday and “here we are”, he told reporters.

Mr Hamilton-Smith said he and the premier were of one mind on the need for stable government.

“We want to build things; we don’t want to rip them down. And I can’t see how the uncertainty and the political point-scoring over coming months and years will help kids, the pensioners,” he said.

The MP said “we are all here to serve” and that he felt he could better do that as part of the government.

“What I am focused on making a contribution,” he said.

“Yes, it’s been a tough decision, it’s the right decision.”

Mr Hamilton-Smith’s move gives the state Labor government extra support in parliament.

Mr Weatherill said the agreement with Mr Hamilton-Smith ensured his backing in key parliamentary votes such as supply and no-confidence motions.

“Mr Hamilton-Smith will strengthen the government in South Australia,” he said.

“I think, on any view, he is widely regarded as one of the most talented members of the opposition.”

Mr Hamilton-Smith will be sworn in by the governor on Tuesday afternoon.