By Paul Lewis
BBC Radio 4's Money Box
Standard Life members who vote next month for the mutual organisation to become a stock market company have been warned they could be worse off within four years.
The insurer has just set out its timetable for the vote
Labour MP Andy Love said research done for his All Party Committee shows that mutual building societies and insurers which convert, pay lower returns after the change.
And he told BBC Radio 4's Money Box that any windfall members may get on conversion soon disappears in lower returns and higher costs.
"We found on average it took about four years for people to lose the benefit that they had received from the demutualization," he said.
Nearly two and half million Standard Life customers will be able to vote on the company's future at a special meeting in Edinburgh on 31 May or by post a few days before.
If three quarters of those voting say "yes", Standard Life could convert to a company by the summer.
All eligible members would then get a windfall payment, representing their share of the value of the mutual.
But Standard Life will not say yet how much the payment might be.
John Hylands, the Standard Life director in charge of the change told the programme: "Each eligible member will get a fixed allocation of shares and most will get a variable allocation as well.
"All other things being equal, a large policy will be entitled to more shares than a small policy. And a policy that has been running for a long time will be entitled to more shares than one that has been taken out recently."
He would not comment on predictions that the average windfall would be £500 to £1,000.
All members will be told by the end of April how many shares they will be allocated.
Standard Life will give an indication then how much the shares might fetch if sold.
But Mr Love warned that some would benefit more than others from the change.
"Some members will get a fixed amount [of shares] and those policyholders may well not benefit, but someone who has been with the society a long time may get a lot of shares and may well benefit." he said.
And he added that in the long term even they may lose out if they stay with the company.
"If they look objectively, the majority will find if they stay for the future they will lose the initial [windfall] benefit they receive from demutualisation and will receive lower returns from a company."
And he said that his research, done by Nottingham Business School found that the people who gained most were not the customers at all.
"It indicated that one of the biggest beneficiaries of demutualisation were directors and senior management of the company."
BBC Radio 4's Money Box was broadcast on Saturday 1 April, 2006 at 1204 BST.
The programme was repeated on Sunday, 2 April, at 2102 BST