Britannia and Yorkshire building societies are slashing minimum balances for new sav ings accounts to below pre-carpetbagger levels.
Both societies are lowering the minimum balance for new instant-access accounts from £2,000 to £100.
As a safeguard against carpetbaggers, the two mutual building societies are demanding that new members sign over any future windfall shares to the societies' charitable trusts.
If either society converts within the next five years, new savers will give up their windfall.
Britannia chief executive John Heaps says: "We are not interested in opening acc ounts for people who are looking for a one-off windfall, which would only make our savings and mortgage rates less competitive over the long term.
"We think the five-year rule will deter short-term spec ulators from disrupting our business and allow us to concentrate on delivering competitive services to people who genuinely want to belong to a mutual society."
Both Britannia and Yorkshire deny their moves are a precaution against new speculation about conversions.
Yorkshire chief executive David Anderson says: "Our philosophy is simple. We use the financial advantages we have as a mutual to offer the most attractive rates possible to our members. That allows us to grow the society more quickly than its costs, making us more efficient."
Britannia's charitable trust is the Britannia Building Society Foundation while Yorkshire's is the Yorkshire Building Society Charitable Foundation.
Minimum opening balance for Britannia's Higher Saver, Capital Trust and Capital Trust 30 accounts will be reduced to £1,000 from £5,000. It is also reopening its Regular Saver Account, its Flexible Savings Account and Flexi-Tessa.