An independent MP has backed Labor to form a minority government in South Australia.

South Australia has avoided a constitutional crisis and the potential of a fresh election with independent MP Geoff Brock delivering Labor the numbers to form a minority government.

Labor will be returned for a fourth term thanks to Mr Brock’s support which ends a week of uncertainty after the March 15 poll returned a hung parliament.

Premier Jay Weatherill says the regional MP’s decision offers the state security and stability for the next four years.

But Opposition Leader Steven Marshall says Labor cannot argue that it has a mandate to govern.

In return for his backing, Mr Brock will serve as regional affairs and local government minister in the next parliament.

He said the decision to back Labor was a difficult one but the right one to ensure government stability.

“I know that it’s not a decision that will please everybody,” he said.

“But my sole motivation has been to arrive at a decision that is right for South Australia and the stability of this state.”

Mr Brock said he was also moved to make a quick decision after it emerged that fellow independent MP Bob Such, who was to share the balance of power, was in hospital awaiting surgery and was expected to be on sick leave for at least two months.

As counting in the election was completed on Sunday, Labor held 23 seats, the Liberals 22 with two going to the independents.

If Dr Such remained unable to support either party, then Mr Brock’s decision was crucial.

Had he opted to back the Liberals, both major parties would have been left with 23 votes on the floor of the parliament.

The likely ensuing chaos could have forced another election, something Mr Brock said was not in the state’s best interests.

In his decision, Mr Brock has pledged to support the government on issues of confidence and supply but will remain free to vote against Labor on other issues.

Mr Weatherill said Mr Brock’s decision offered South Australia secure and stable government and his inclusion in the cabinet would provide a strong voice for regional South Australia.

“Mr Brock and I share one crucial thing in common and that is our love of South Australia,” the premier said.

“I’m very confident that Mr Brock being part of our team will add an extra dimension to our capacity to represent every South Australian.”

Mr Marshall said that while he accepted Mr Brock’s decision he did not believe Labor had a mandate for its policies.

“Labor has no mandate whatsoever,” he told reporters.

“They dropped their vote, they dropped three seats and lost three ministers at this election.”

Mr Marshall said he did not expect any move against him from within the Liberal Party following the election outcome.

Federal Labor leader Bill Shorten congratulated Mr Weatherill on forming government, saying Prime Minister Tony Abbott had to share some responsibility for the Liberal Party’s failure.

He said Mr Abbott’s policies on auto manufacturing and his refusal to release the government’s commission of audit had hindered their campaign.

“This election proved that there is no such thing as an unwinnable or an unlosable election,” Mr Shorten said in a statement.

“This is a shock defeat for the Liberal Party.”