By Paul Lewis
BBC Radio 4's Money Box
Cheaper mortgages, lower charges, and better savings deals have been promised by the boss of Britain's biggest building society.
Mr Williamson was speaking about his society's merger plans
Nationwide chief executive Philip Williamson was explaining more about the merger of his society with the smaller Portman.
He said nothing was "guaranteed" but he "confidently expected" that outcome, which will see 1.8m Portman members getting a windfall of at least £200.
Mr Williamson also said that with the existing boards there was "absolutely no chance" of the new Nationwide changing its status from a mutual building society.
The merger between Britain's first and third biggest building societies will create a giant organisation with 13m members and assets of £150bn.
It will be the second biggest mortgage lender in the country, beaten only by Halifax Bank of Scotland group.
Matthew Wyles, a director of Portman, told BBC Radio 4's Money Box that if his members voted for the deal they would get a share of £500m "that will mean a minimum of £200 [each] and we believe the average will be substantially in excess of that".
Nationwide confirmed that its members would not have a vote on the merger as the Financial Services Authority has already agreed that the board could make the decision.
Philip Wiliamson explained why his members would not get a windfall: "Portman is a very successful society and on day one our members will benefit from a massive profit stream of circa £100m which will come straight into the Nationwide coffers.
"We will utilise that to deliver better value to our members. It is not appropriate for us to be distributing payouts to our members.
"We believe the benefits will enable us to make mortgages cheaper, provide better savings rates, and fewer fees and charges than we do today. I couldn't use the word guarantee but we confidently expect that this will be the outcome of the deal."
And he reiterated that Nationwide was not going to follow 10 other societies that had become banks or been bought by a bank.
"There is absolutely no intention within the existing boardroom to be anything other than a mutual building society," he said.
"There is absolutely no chance whatsoever provided the existing boards continue with their existing policies.
"I can't say 20 years from now they won't have a different policy."
Both societies also confirmed to Money Box that members of Lambeth Building Society - which is currently being taken over by Portman - would be treated like other Portman members and would normally be eligible for a second windfall from the Nationwide merger.
BBC Radio 4's Money Box was broadcast on Saturday, 16 September, and repeated on Sunday, 17 September, 2006 at 2102 BST.