Roger Thayne has said 500 lives a year are at risk

Roger Thayne, who dramatically resigned in May, warned 500 lives were at risk each year unless there was investment. This prompted calls for the inquiry.

Mr Thayne said he believed the situation in the ambulance service was "dire" and that �35m was needed.

A voting blunder by Health Minister Brian Gibbons helped secure the inquiry he strongly opposed.

I think that questions have to be asked about the management of the ambulance service over the past five or six years Roger Thayne, Ex-ambulance service head

His mistake happened at the end of a debate, called by Plaid Cymru, at the Senedd on Tuesday.

It followed months of turmoil at the ambulance trust which has seen Mr Thayne quitting on top of failures to meet emergency call-out targets.

Speaking out last month, Mr Thayne said he could not work for a service he felt "ashamed" of and did not want to be accountable for an organisation that was failing so badly.

He had written a damning report calling for �35m investment and catalogued outdated equipment, a history of poor management and delays in getting ambulances to patients, which he said contributed to a "dangerous" service.

He said he resigned after he was asked to make cuts.

Dr Gibbons was against an inquiry but voted for it by mistake

Reacting to news of a public inquiry, he told BBC Wales: "I believe that the situation was so dire that we needed a public inquiry to address why so much money had been wasted in such a very, very important public service.

"I think that questions have to be asked about the management of the ambulance service over the past five or six years."

He also questioned the validity of a single ambulance service for Wales.

At the vote for a public inquiry on Tuesday, Assembly Members voted 28 to 26 in favour of an inquiry - this included Dr Gibbons voting by mistake with the opposition, while his predecessor Jane Hutt failed to vote with Labour.

Dr Gibbons pressed the wrong button, but Presiding Officer Dafydd Elis-Thomas told him that there was nothing he could do.

While the debate continues... it is our priority to progress the plans to modernise the service for the benefit of the people of Wales Ambulance trust interim chief executive Anton van Dellen

Ms Hutt, now the business minister, later complained her vote against the motion was not registered.

The assembly government also said it was "far too early" to estimate the cost of a public inquiry.

There was further controversy after First Minister Rhodri Morgan said Mr Thayne had not been in post long enough to understand the problems facing the ambulance trust and that the pressures of commuting from Staffordshire had contributed to his resignation.

Mr Thayne has dismissed the claims and opposition members have called for Mr Morgan to apologise.

In a statement issued later, a spokeswoman for Mr Morgan said: "The first minister made his comments on the situation as he understood it. There was no intention on his part to slight Mr Thayne."

Meanwhile, a joint statement from ambulance trust chairman Stuart Fletcher and interim chief executive Anton van Dellen, said they would co-operate fully with the inquiry.