RENEWED calls have been made for the health service to re-investigate Scotland's waiting times scandal following the departure of its chief executive.

The Conservatives led the calls as it emerged Derek Feeley is to step down in the summer as chief executive of NHS Scotland.

He is due to take up a post as executive vice-president of the Institute for Healthcare Improvement (IHI), based in Massachusetts.

Mr Feeley, who is also the Scottish Government's director-general for health and social care, was recently challenged by watchdogs over evidence he gave to MSPs on the handling of waiting times.

As the NHS's top official he has staunchly defended the service against claims that abuse of the waiting times system was widespread, after it emerged one health board, Lothian, had manipulated figures by unfairly declaring patients unavailable for treatment.

The Scottish Conservatives called on his replacement to launch a fresh investigation.

Health spokesman and deputy leader Jackson Carlaw MSP said: "There are still very blatant questions to be answered on the waiting list scandal, not least why Mr Feeley's evidence to Holyrood differed so greatly from Audit Scotland's. The Scottish Government has tried on numerous occasions to brush off this fiasco, but that will not be allowed to happen until we get answers.

"I hope Mr Feeley's successor will not only guarantee this disgraceful practice will not be repeated, but also appreciates the issue needs to be revisited."

Mr Carlaw said there remained too many unanswered questions and the patients and staff who had suffered due to the scandal deserve to know what they are.

Giving evidence at Holyrood earlier this year, Mr Feeley denied health boards across Scotland suddenly stopped declaring patients "socially unavailable" after the Lothians scandal became public.

However, his evidence was questioned by Auditor General Caroline Gardner.

The public spending watchdog said the proportion of patients listed as being unavailable only decreased slightly in 2010 before rising again. It added there was only a marked and sustained decrease the following year.

Health Secretary Alex Neil paid tribute to Mr Feeley's "visible leadership, foresight and dedication" as his departure was announced.

He added: "He has made a major contribution to health and healthcare in Scotland and I have no doubt he will bring the same drive to his new role."

Mr Feeley, who has led NHS Scotland since 2010, said: "I would like to thank my colleagues in Government and in the health service for all of their help and support over the years. It is a great honour to be offered this opportunity with IHI as they work to make health and care better across the world."

There was praise for the outgoing chief executive who has worked in Government service for 30 years.

Permanent Secretary Peter Housden described him as an outstanding leader of the NHS. He added: "He has a broad knowledge of public services and his immense reserves of drive and passion for quality have seen very substantial improvements in services in testing times. He will be much missed."