The so-called bedroom tax, which penalises tenants with spare rooms in social housing, would lead to a rise in people getting into debt and, ultimately, evictions, campaigners have warned.
But now advice agencies say tenants might be able to get around the tax because the law is unclear about what is classed as a bedroom.
They claim if tenants simply ensure spare rooms are not used as bedrooms, local authorities and housing associations can be challenged over any cuts in payments for housing benefit purposes.
The Glasgow Advice Agency (GAA) – a consortium of consumer assistance bodies – obtained the opinion from Jonathan Mitchell, QC, who said that, while it was for a local authority to decide what was a bedroom, there was no legal definition.
The only guidance uncovered was in the Rent Officer Handbook produced by HM Revenue & Customs, which "makes the important point that actual use by an actual household is usually critical".
Mr Mitchell said it would be "going wrong in law" if a local authority determined every room that could possibly be slept in was classified as a bedroom, whatever its characteristics or use.
He added: "It may be that tenants should be advised that the particular use they make of rooms may have consequences for their benefit.
"If, for example, a family with a disabled child allows him or her to use the living room as a bedroom, this may result in the property being determined to have one more bedroom than before, just as turning a bedroom into a therapy or care room, or a study or playroom, may result in a reduction in the number of bedrooms determined."
Part of a raft of welfare reforms taking effect from April, the bedroom tax was designed to cut £500 million off the £21 billion bill for housing benefit. In Scotland, it is expected to save £50m from an annual housing benefit bill of £1.7bn.
Working-age tenants in housing association or council homes will lose 14% of their housing benefit entitlement if they have one spare bedroom, and 25% if they have two or more spare.
Yesterday, a leading Nationalist MSP indicated the SNP would scrap the UK Government's controversial cut if Scotland became independent.
Mike Dailly of GAA, who is also a member of the Secretary of State for Scotland's Poverty Advisory Group, said tenants could mount a legal challenge against attempts to impose the bedroom tax if rooms were not being used as bedrooms. He added: "This is good news. This has thrown a lifeline to many of the Scots who are affected by this.
"GAA believes it is possible for tenants to change the use of what might be regarded as a spare bedroom into something that need not be counted as a bedroom, and therefore not be subject to the 14% cut in housing benefit.
"This is of major application for disabled tenants, who may use a 'spare bedroom' for the purpose of therapy, storing wheelchairs, medical equipment or to undertake medical procedures. Other tenants may be able to avoid the bedroom tax too.
"We are calling for the Scottish Government to produce its guidance for local authorities, which could give us a bit of consistency."
The "under-occupation" rules were expected to affect foster carers with rooms spare between placements, disabled people who sometimes need a carer to stay overnight, separated parents who use a spare room for child access, and the homes of soldiers on tours of duty.
The Convention of Scottish Local Authorities (Cosla) said up to 20% of Scotland's 315,000 council tenants would be hit, and that 40% of them would get into rent arrears – leaving councils with a £20m-a-year shortfall, as well as the extra cost of enforcing debts and evictions.
A Cosla spokesman said local authorities would have to examine the legal opinion before adopting a position.
The Department for Work and Pensions (DWP) said it tailored housing benefit to the way the social landlord or local authority defined the property.
A spokeman said: "If a social landlord says it is a two-bedroomed house, regardless what happens to it subsequently, it is a two-bedroomed house and that is what housing benefit will be judged on."
Speaking on radio yesterday, SNP MSP Clare Adamson said the proposed cut to housing benefit, based on spare bedrooms, was an "appalling piece of legislation".
She identified the cut as one which could be reversed by a future SNP Government if Scotland became independent. However, she said not all UK Goverment welfare reforms would be abolished.
Her comments prompted claims by Scottish Labour that the SNP was trying to bribe voters to back independence while failing to use existing Holyrood powers to mitigate the impact of welfare reform.