SCOTS homeowners could be plunged into a "spiral of debt" as banks hike up mortgage repayments for tens of thousands of borrowers from today, campaigners have warned.

Customers with the Halifax – the UK's biggest mortgage lender – could find themselves paying around £200 a year extra, following rate rises. The increases will affect borrowers on standard variable rate (SVR) mortgages and those on fixed-rate deals due to expire.

Halifax is raising its SVR from 3.5% to 3.99%, an increase that will affect 850,000 UK homeowners who will be paying an average of £16.40 a month more in repayments.

The Co-operative, Clydesdale and Yorkshire banks are also raising their rates from today, blaming the weak economy and the increased cost of funding a mortgage.

Graeme Brown, director of Shelter Scotland, said the rate rises could push thousands of struggling homeowners to the brink, with calls to their repossession hotline already soaring last year. He said: "News that some lenders are going to put up interest rates will set off alarm bells in many thousands of households across Scotland.

"We have already seen a 40% increase in calls to our legal service on repossessions and, of the more than 10,000 calls taken by our free helpline advisers last year, many needed money advice on how to make ends meet. My fear is that even a 0.5% increase in some mortgage rates will push many over the edge into a spiral of debt, arrears and eventually repossession."

Around 54,000 Co-operative Bank customers will see SVR rates go up by 0.5% to 4.74%, meaning payments will typically increase by £15 a month, or £180 a year.

Clydesdale and Yorkshire banks' SVR rate will rise from 4.59% to 4.95%, affecting 30,000 customers. RBS-Natwest is also pushing up the rate on its One Account, a non-SVR product, by 0.25%, affecting 100,000 customers.

The rate rises have sparked fears people could struggle to switch to a better deal as lenders have started tightening their borrowing criteria, triggering a fall in mortgages being approved.

Borrowers with a poor credit rating, high loan-to-value ratios, or who are in negative equity could find themselves stuck with their existing lender and unable to switch to another provider, consumer campaigners warn.

Mike Dailly, a solicitor with Govan Law Centre who also sits on the Financial Services Authority's (FSA) consumer panel, said that, while the rate rises were "relatively small", he feared there could be serious consequences from the cumulative effect of several increases further down the line.

He said: "It can still be very difficult for people who have got very tight budgets and don't really have any money to spare, but the reality is the increases are still relatively small.

"The fear that we [on the FSA consumer panel] have is that this could just be the start of it. If you consider the proposition that many people are trapped in these mortgages and we have a double-dip recession, which means things aren't necessarily going to get any better, especially if the Bank of England raises its base rate, then that's when we're really going to see much more of an impact."

However, the Council of Mortgage Lenders said it has seen "little evidence so far" that borrowers on an SVR are paying uncompetitive rates. Its analysts insist market pressures will keep rates in check as lenders who raise SVR rates too aggressively risk losing their most creditworthy customers.

Ray Boulger, senior technical manager at mortgage adviser John Charcol, said the degree of equity borrowers have in their home will be "the most important factor" for those facing rate rises and who want to switch deals.

He advised those worried they may become "trapped" to increase equity by overpaying their mortgage if possible, pointing out that previous falls in the variable rate meant many homeowners were actually paying less per month now compared with a few years ago.