By Paul Lewis
BBC Radio 4's Money Box
Windfalls could return next year if a prediction by business consultants Deloitte comes true.
Four mergers of building societies were announced in 2006
It says that 10 building societies could merge into five in 2007 releasing windfall payments for members of up to £500.
When societies merge, the members of the smaller one get a tax-free payment to reflect their loss of membership.
In 2006, four mergers were announced yielding windfalls of between £100 and £400.
It is already known that nearly two million customers of Portman will get at least £200 if the merger with Nationwide goes ahead in Autumn 2007.
Stephen Williams, head of building societies research at Deloitte, told BBC Radio 4's Money Box programme that there could be more.
"There's been four announced over the last year, that's eight societies coming together. We're expecting that to be 10 in the next year," he said.
He would not name the societies he expected to be taken over but thought it would be: "among the smaller societies where the chief executive is nearing retirement, the board can't face the regulatory and other challenges and therefore would happily merge. Among the larger societies it is trying to increase their franchise and broaden their range of products".
And the societies which acquired them he expected to be: "focussed on the top 20 societies.
"We would expect windfalls to occur. Somewhere between £200 to £500 would be a reasonable benchmark."
'Time of change'
But the Building Societies Association, which represents the 60 remaining societies in the UK, was more cautious.
Director general Adrian Coles told the programme that five mergers was "on the high side" as there had been none in 2004 or 2005.
But he admitted: "It is a time of change. Markets are becoming more competitive and building societies are leading that because of course they don't have dividends to pay to shareholders.
"But inefficient providers - building societies or banks - might find it a bit tougher in the next year or so," he added.
He also denied that Nationwide - now as big as all the other societies put together - would leave the movement and become a bank.
"It looks to me very unlikely that in the next five years Nationwide will become a bank," he said.
"The other building societies would survive if it did but it seems very unlikely that Nationwide is going to take that step in the near future."
He echoed the comments made by Nationwide chief executive Philip Williamson on Money Box in September 2006 when the merger was announced.
Asked if there was any chance of Nationwide becoming a bank Mr Williamson replied: "Absolutely no chance whatsoever provided the existing boards continue with their existing policies.
"I can't say 20 years from now that they will not have a different view."
BBC Radio 4's Money Box was broadcast on Saturday, 23 December at 1204 GMT.