In a move which has been privately welcomed by jail bosses and prison unions, a leading lawyer will seek compensation for prisoners who have had to "triple up", claiming the practice is degrading.

Some senior insiders at the Scottish Prison Service (SPS) and the Scottish Prison Officers' Association (SPOA) are privately sympathetic to the legal case because they believe it will increase pressure on ministers to solve the overcrowding crisis.

The lawyer involved, Tony Kelly, was last night jokingly referred to by one prison service boss as "the best friend of the Scottish Prison Service".

Previously, Kelly scored a massive legal victory by forcing most prisons to end "slopping out" on the basis it breached human rights. That case forced ministers to make payments of thousands of pounds to affected prisoners but also increased funding to jails to help deal with the underlying problem.

Kelly told Scotland on Sunday: "It is sad that is has come to this. I take no pleasure in running off to court. This is a measure of how far we have come, that prison numbers rise and rise and we end up with legal action."

Kelly said overcrowding in jails is now so bad it amounts to a clear breach of the Government's legal obligations to prisoners that they should not be held in inhumane or degrading conditions. He said: "The legal basis for action is crystal clear."

Although the case is about three to a cell, Kelly believed inmates forced to "double up" for long periods would also have grounds to sue.

SPS chief executive Mike Ewart last week told MSPs his prisons were so overcrowded he was looking at emergency plans to make some inmates sleep on mattresses in gyms.

He also revealed he may be forced to ask ministers to authorise an emergency release of prisoners in the event of a serious problem at a jail, such as a fire or flood, because there is simply no spare capacity in the system.

There were 8,080 inmates in Scottish jails on Friday. That is about 2,000 more than the prisons were built to take and just 46 short of an absolute safe limit set by prison authorities.

No prisoners were believed to be tripled up over this weekend.

An SPOA source told Scotland on Sunday he hoped the threat of legal action would prompt ministers to act to stop "tripling up", although he admitted this was "an ironic thing for me".

Inmates winning compensation is a frequent source of irritation to many prison officers but the source added: "Overcrowding makes our job a lot more difficult and much more dangerous."

The SNP has increased capital spending on prisons to £120m this year. A new prison, Addiewell in West Lothian, should be operating by spring. Justice Secretary Kenny MacAskill says sentencing reforms must bring down numbers.