Martin Hamilton-Smith has been branded a traitor after he quit the Liberals to become a minister in SA’s Labor government.

A former South Australian Liberal leader has joined the state Labor government as a senior minister, prompting cries of political betrayal and treachery from his ex-colleagues.

After declaring himself an independent Liberal, Martin Hamilton-Smith made his shock announcement on Tuesday, saying he was putting the people of South Australia ahead of politics.

In return for becoming Minister for Investment and Trade, Defence Industries and Veterans’ Affairs, he will support the government in key parliamentary votes, such as supply and no-confidence motions.

Labor holds 23 of the 47 seats in Parliament, but it can now rely on Mr Hamilton-Smith as well as independent, Geoff Brock, who has already been appointed regional development minister.

Opposition Leader Steven Marshall described Mr Hamilton-Smith as a traitor, saying he was the fourth Liberal approached by the ALP in a bid to shore up “this hopeless, tired, dysfunctional” government.

Mr Hamilton-Smith, who attended a shadow cabinet meeting and Liberal Party branch meetings on Monday, told reporters that SA needed certainty and stable government.

He said he and Premier Jay Weatherill, who smiled broadly during the announcement, were of the one mind.

“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, those who might be worried about their small businesses surviving,” he said.

But Mr Marshall, who cancelled a trip to New Zealand when he heard the news at the airport, said Mr Hamilton-Smith had betrayed his conservative electorate of Waite, an Adelaide seat he had held since 1997.

“Mr Hamilton-Smith has betrayed small businesses in South Australia for now adopting Labor policies that will hurt businesses going forward,” he said.

His betrayal and treachery was unrivalled in South Australia’s political landscape.

“Martin has made this decision in his own interest; it is a despicable decision,” he added.

“Martin Hamilton-Smith should hang his head in shame.”

But while the defection had been a “body blow”, he said the Liberal Party would pick itself up and hold the government to account.

Mr Hamilton-Smith denied betraying voters in his electorate, saying they elected a Liberal and “they still have one”.

“Steven and others in the Liberal Party will be disappointed with my decision today, there is no question, and friends I have had for a very long time I will lose, I accept that,” he said.

AT the end of the day “we are all here to serve” and he felt he could better do that as part of the government.

Premier Weatherill said the decision drew on the best available talent available in SA to strengthen the government.

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

Mr Weatherill said obviously in politics there had been differences of opinion, but he and Mr Hamilton-Smith both shared a love of SA and a commitment to its future.