Christine Graeme raised concern over the figures

The average number of days spent in custody for offences including rape and lewd behaviour in 2005-06 was 1,038, according to official statistics.

Nationalist MSP Christine Grahame, who obtained the figures, called for a review on sentencing guidance.

However ministers said sentence lengths based on a relatively small number of cases were expected to fluctuate.

The figures, obtained through a parliamentary question, also took in sentences for indecent assault, which included attempted rape, and "other crimes of indecency".

AVERAGE TIME IN CUSTODY FOR CRIMES OF INDECENCY 2001-02: 1,203 days 2002-03: 1,253 days 2003-04: 1,081 days 2004-05: 1,295 days 2005-06: 1,038 days click here and scroll down to read the figures in full detail (external website)

The statistics, which excluded life sentences, showed fluctuations in the average number of custodial days served for indecent crimes to be relatively small.

The figure stood at 1,203 in 2001-02 and rose and fell in subsequent years.

Despite this, Ms Grahame, who represents the south of Scotland, said: "These figures highlight a downward trend in the length of time convicted sex offenders are spending in jail and I think that sends the wrong message to victims of such crimes, in terms of encouraging them to come forward to report incidents.

"Equally I believe it sends entirely the wrong message to both perpetrators and victims that such offences are not being dealt with as firmly as they once were."

A statement by community safety minister Fergus Ewing accompanying the figures said: "Average sentence lengths based on [a] relatively small total number of cases can be expected to fluctuate from year to year."