Hospitals are facing an increasing shortage of consultants, with figures showing the number of jobs lying empty for six months or more has doubled in the last year.

The number of consultant posts that are unfilled has more than trebled in the last four years, with the current vacancy rate standing at 8.3%, according to NHS statistics.

In June 2011 the NHS reported that 127.9 whole time equivalent (WTE) consultant posts were vacant, but by June this year that had risen to 447.5 WTE.

Of these, 188 WTE positions have been empty for at least half a year, a rise of 102.4% on June 2014.

Health Secretary Shona Robison said any rise in vacancies is "in part due to the the creation of new posts in health boards".

But the figures sparked a call for action from opposition MSPs at Holyrood.

Liberal Democrat health spokesman Jim Hume said: "The Health Secretary must set out the Scottish Government's plans to tackle this skills shortage.

"With vacancies for consultants trebling in four years, health boards will be forced to rely on more expensive and short-term solutions such as the use of locum doctors. The increase in posts lying vacant for more than six months is particularly worrying.

"The SNP government must work with health boards and local authorities to find innovative ways to recruit and retain consultants."

Scottish Conservative health spokesman Jackson Carlaw said: "It's very clear that, on the SNP's watch, the number of vacancies across Scotland's NHS has increased.

"This is not a new issue and it's one the Scottish Government should have got on top of long ago.

"Of course, ministers cannot simply create applicants to take these jobs, but it's pretty obvious Scotland is not training enough of its own specialist NHS staff."

The high number of vacancies can "arise to meet the increasing demand of the health service", the latest NHS workforce report said.

Overall staffing in the NHS has risen by 4.2% over the last four years, with 137,420.7 WTE staff employed by the health service in June 2015 - an increase of 5,506.8.

The total includes 59,073.6 WTE nursing and midwifery staff, an increase of 611.4 on June 2014.

The number of WTE consultants has also risen over the last year to 4,943 - up by 252.7 on 12 months ago.

At the same time, the number of nursing and midwifery jobs that are vacant has increased by 390.6 WTE posts to 2,255.9 - giving a vacancy rate of 3.7%.

Ms Robison said WTE staffing levels in the NHS had increased by more than 10,000 since the SNP came to power in 2007.

The Health Secretary said: "Today's figures show that the number of people working in NHS Scotland has increased by more than 10,000 under the current government.

"This includes more than 2,000 additional WTE-qualified nurses and midwives working in Scotland's NHS, and an additional 1,300 WTE consultants.

"The recent rise in nursing and midwifery vacancies is largely a reflection of the creation of new posts in health boards, which reflects the needs of patients.

"Several health boards have received additional investment which has allowed them to increase their nursing numbers and are in the process of recruiting these extra nurses.

"There has also been a rise in consultant vacancies during this quarter, which is again due to the creation of new posts. It is expected that while additional posts are created there will be an increase in vacancies as boards recruit."

Dr Nikki Thompson, chair of the British Medical Association's Scottish Consultants Committee, said: "Official figures show that the number of vacant consultant positions in Scotland has risen by almost a third in the last year.

"We also know that there are consultant vacancies that do not get picked up in official statistics, so the real picture is very likely worse.

"Consultant care must be available when patients need it. We know that remaining consultants are struggling to cover the gaps, but there is a limit to how long people can work safely at this intensity.

"Twice as many consultant jobs are now lying vacant for months on end, and staff and services cannot continue at this level of pressure.

"The Scottish Government must recognise that they have a major recruitment and retention problem, which will damage the care that patients receive.

"They must take urgent action to value and retain those consultants we have, and to attract the others that patients and services need."

Theresa Fyffe, Scotland director for the Royal College of Nursing (RCN), said: "According to these latest workforce figures, the number of nursing and midwifery staff in the NHS continues to rise.

"On the face of it, this is good news, but we know that the demands on our health services are outstripping this latest increase: our population is getting older, with many now living with a number of long-term conditions, meaning that more and more people are needing ever-more complex care, and the costs of new drugs and technology are sky-rocketing.

"If we are going to meet this growing demand on our health service, brave decisions have to be made now on how services are to be delivered in the future."

Ms Fyffe insisted the "status quo is not an option, given the pressures the NHS is now under" and added: "Hiding behind soundbites such as there are more nurses now than a few years ago is really not good enough when the latest NHS staff survey shows that only a quarter of nurses and midwives (25%) think there are enough staff to do their job properly.

"If the NHS is to flourish into the future, we need to take a long, hard look, with the public, at how we deliver services differently to put the NHS workforce on a sustainable footing to ensure quality care for patients into the future."