PEOPLE will be "tripping over" the homeless as a result of deep cuts to housing benefit which start in a few weeks' time, the leader of Scotland's councils has warned.

David O'Neill, president of the council umbrella group Cosla, said he feared 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.

He said the policy, which will leave up to 100,000 working age tenants with bills of between £50 and £90 a month, seemed geared towards problems in London and would backfire badly in Scotland.

The growing political row over the bedroom tax dominated Prime Minister's Questions last week, as Labour leader Ed Miliband quizzed David Cameron about individual cases of hardship.

Part of a raft of Coalition welfare reforms taking effect from April, the bedroom tax is 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.

The Government expects those affected to find the extra by finding jobs or working longer hours.

If they can't afford to pay, tenants are advised to find smaller homes or take in a lodger. If people move out of larger homes, they argue, it should free them up for families who are over-crowded.

However, the Department for Work and Pensions (DWP) admits there is not enough one-bedroom social housing to accommodate those affected. Critics say the change is a recipe for disaster – tipping people who cannot move into debt, or pushing them into the private sector where rents are higher, resulting in higher benefit bills.

The "under-occupation" rules will 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 who are on tours of duty.

Cosla estimates up to 20% of Scotland's 315,000 council tenants will be affected, and that 40% of them will get into arrears, leaving councils with a £20m a year shortfall, as well as the extra cost of enforcing debts and evictions.

O'Neill, former Labour leader of North Ayrshire, stressed he was not opposed to benefit reform, but said this change could generate greater demands for welfare services.

"Think of the mother whose son is in Afghanistan and comes home once every six months," he said. "Has she to get rid of his bedroom and he can't come home?

"What about parents who are separated and want to have a continuing relationship with their kids? Do they get the bedroom taken off them, thereby jeopardising the relationship with the kids?

"The whole thing has been badly thought out, and our fear is it's going to be badly implemented.

"I get the train in the morning, and I walk from [Glasgow] Central Station to Queen Street Station, and The Big Issue sellers are there. This time next year, I'm probably going to be tripping over them. I think it's going to prove to be disastrous."

O'Neill said the policy appeared driven by London's needs, where 16% of tenancies are overcrowded and only 19% have spare bedrooms.

In Scotland, there is less overcrowding and in some councils such as East Dunbartonshire, more than 50% of tenants have a spare bedroom.

In response, the Coalition announced an extra £30m in Discretionary Housing Payments (DHPs) for councils – £5m for foster carers and £25m for disabled people to stay in their adapted homes.

But Stephanie Stone, assistant director for family placement at Barnardo's Scotland, said there was little detail on how DHPs would work, or even which council would pay if foster carers took in children from distant local authorities.

She said: "For those carers on housing benefit, it's a great source of anxiety.''

David Ogilvie, policy manager at the Scottish Federation of Housing Associations, said social landlords would try to work with tenants to clear arrears but also said housing associations wished the bedroom tax didn't exist.

He said: "It's unfair on tenants because it's not their fault there's no small housing stock or sufficient DHPs.

"It's going to impact on vulnerable people, people in adapted homes, disabled people, and it's unfair to landlords because it's putting their income at risk"

Defending the Coalition's policy, Scottish LibDem leader Willie Rennie said: "No change is not an option. We're conscious of issues such as the lack of one-bedroom poperties and divorced parents having access to their children, and we are working on them."

Cosla president predicts surge in homelessness