The acting chief executive of the Welsh Ambulance Service Trust, Roger Thayne, has resigned only two months after taking up the post.
Roger Thayne said his resignation was 100% his own decision
His departure comes as figures show ambulance emergency response times in Wales are failing to meet targets.
Welsh Lib Dems said it threw the service into "disarray and crisis," and Conservatives called it "astonishing".
Mr Thayne, 62, said his retirement had nothing to do with the figures, which he had not known were being released.
He said that "an earthquake" was needed to change the way the service was working.
Statistics released by the assembly government up to the end of March, show the percentage of responses to emergency calls within 21 minutes was at 81.4%, well short of the target of 95%.
Referring to response times, Mr Thayne said: "They will always be bad until you change the system. I only came to help out in the interim."
Lib Dem health spokeswoman Jenny Randerson said his comments needed to be "taken very seriously".
"This throws the ambulance service into total disarray and crisis because it is against a background of failed targets and poor response times over many years," she said.
'Trying to save lives'
"When Roger Thayne took up his post just two months ago he was hailed as the man to sort out the problems. He's come and looked and realised that the problem is beyond him."
Mr Thayne joined the Welsh Ambulance Service after resigning from its counterpart in Staffordshire in March.
Asked about his relationship with staff, he told BBC Wales that his "way of leading people is to look them in the eye", and that he didn't recognise staff as management but as workers for the service, trying to save lives.
He had been met with criticism from health unions who said relations with the service had become worse since Mr Thayne's appointment.
One of the most controversial changes was a cut on overtime.
Ambulance response times in Wales are failing to meet targets
Mr Thayne defended that change, saying overtime had never been halted but had to be managed better due to a pay increase for staff adding to overall costs.
Welsh Health Minister Brian Gibbons said another acting chief executive was likely to be appointed, ahead of the appointment of a permanent chief executive - a post which is currently being advertised.
Dr Gibbons added: "I understand that Mr Thayne, who joined the ambulance trust as acting chief executive on a monthly renewable contract earlier this year, has indicated today that he will not be extending his time in Wales beyond the end of this month."
Mr Thayne resigned from his Staffordshire post in March over the creation of a new regional emergency service.
In Staffordshire, he had succeeded in ensuring that 90% of life-threatening emergencies were reached by ambulance crews within eight minutes.
Before that he spent 32 years in the military establishing medical support to the Falkland Islands post conflict, medical disaster relief to eastern Nepal, Lockerbie, and the 1989 Kegworth plane crash.
He also played a role in medical plans for the Gulf War.
Dr Anton Van Dellen, who came to the Welsh Ambulance Service Trust with Mr Thayne from Staffordshire, has remained with the trust as director of operations.