The union representing rank and file police has written to the Scottish Government over growing concerns that a scheme aimed at retaining experienced officers is being phased out.

The police federation for Strathclyde, Scotland's largest force, has contacted Kenny MacAskill, the Justice Secretary, to ask how he hopes to fulfil his controversial promise of putting 1000 additional officers on the streets if forces cannot afford the retention scheme.

Across Scotland there are approximately 155 officers on the 30-plus scheme, an arrangement which allows them to stay on beyond the usual 30 years service without losing out on their pensions. More than 100 are with Strathclyde.

Those on the scheme retire for one day before returning to the force on the same salary and are allowed to claim their lump-sum pension at this stage.

However, in the past two weeks the Strathclyde Police Federation has been contacted by more than a dozen officers on 30-plus who have been told their contracts will not be renewed. Several are understood to be appealing.

In their manifesto the SNP pledged to introduce 1000 new police to communities. However, since then the Justice Secretary has rephrased the pledge to an effective increase of 1000 officers.

Last year, he talked of the importance of the three Rs: recruitment, retention and redeployment. Ministers have promised to recruit 500 police officers and fill the other spaces by reducing bureaucracy and retaining those due to retire.

Over the next two years some 2500 officers are expected to retire, making retention a key part of just maintaining current numbers.

Raymond Pratt, secretary of the Strathclyde federation, said: "There seems to be a serious discrepancy between Mr MacAskill's pledge to retain more officers and the fact that so many of the experienced officers on the 30-plus scheme are being given 28 days' notice to leave," he said. "We do not blame the chief constable as he is dealing with a limited budget.

"He needs the funding to both recruit and retain officers. There are 171 new recruits waiting to join Strathclyde but we've been given the impression that they cannot come in until the books are balanced.

"More than a dozen people have already been told their contracts will not be renewed. This flies in the face of what the Justice Secretary promised to do last year."

In the letter to the Justice Secretary, the federation asks him how he plans to retain officers without the financial incentive of the scheme.

Chief constables voiced concerns last November that retaining one officer might mean not being able to afford the recruitment of others.

An inspector on 30-plus scheme could earn £47,000 a year as compared with a new probationer on £19,000.

A Scottish Government spokesman said: "The recruitment, retention and redeployment of police officers are matters for which chief constables are responsible. We will support chief constables in this.

"Increased frontline policing is what the public want and expect and what this government is committed to delivering."

Meanwhile, more than 70,000 police staff and community support officers in England and Wales have accepted a 2.5% pay offer.

The rise will be backdated to September and is in contrast to a decision by the government to withhold three months' backdated pay for police officers south of the border, effectively reducing this year's award from 2.5% to 1.9%.

Unison had recommended acceptance of the offer. National officer Ben Priestley said: "Pay negotiations this year have been very tough because of the government's artificial limit on public sector pay.

"I am sure it's been a hard decision for members facing rising fuel, food and housing costs and they will want to see a catch-up deal later this year."