Two sex offenders released into the community in Scotland are being forced to wear an electronic tag - one for the next 10 years - because they are considered such a risk to society.

In what is thought to be the longest tagging conditions ever placed on criminals in the UK, one man has been put on an electronic monitoring order for the next decade and the other for eight years.

The men, who cannot be named for legal reasons, were both serving sentences of more than four years before their release last year. Their risk of reoffending is deemed so high they must remain within their homes next to the electronic monitoring box for at least 12 hours overnight.

For the remaining half of the day, for at least the first six months of their release, they are receiving supervision from Sacro, a charity that works with offenders in the community.

One source close to the decision said there are concerns about tagging people for such long periods.

"There are some who think such a long-term order breaches the civil rights of the offenders but when their risk is considered so high, intensive support must be in place to ensure they do not reoffend," he said. "Others argue if someone is so high a risk they need a tag, perhaps they should not be released at all."

The parole board has had the power to include tagging on release as part of the conditions of licence since January 2004. The board was initially reticent about using the power but in the past few months The Herald has learned some 15 criminals have been tagged on the board's recommendation, two of whom have been recalled to custody for breaking conditions of the licence.

The move follows calls by Cathy Jamieson, Justice Minister, to make use of powers to tag prisoners on release amid concern about the numbers reoffending.

In 2004 she intervened in the row over the early release of prisoners by calling for the parole board to be made more accountable to politicians and victims and to use electronic monitoring.

The parole board of Scotland directs the release of prisoners serving four years or more. It makes release conditions for life sentence prisoners and those on extended sentences.

Once released, offenders on licence have their progress monitored by supervisors who can refer back to the parole board to have conditions changed.

Sandy Cameron, chair of the parole board, said the aim of the board is to reduce the risk to the public as far as possible.

"We would always look at whether there are additional conditions we can put in place to protect the public," he said. "There are quite clear constraints and messages to the licencee and tools for the supervisor to ensure compliance.

"We would expect the supervising officer to come back to us if they felt additional conditions were required."

Professor Mike Nellis, an expert in tagging at Glasgow University, said: "There is a line of argument that says the longer you spend tagged, the more intolerable it becomes. People can put up with staying in under curfew for months but if it extends to much longer than that the risk of breach is greatly increased.

"In parts of the US such as Florida they have started tagging sex offenders for life, but the only way that is going to work is if there is a great deal of flexibility.

"If we are going to breach people for running 10 minutes late it is never going to work."