New rules have come into force which will free up credit unions to compete more effectively with banks.
The changes, under a Legislative Reform Order (LRO), mean the financial co-operatives can now pay interest on savings for the first time and expand beyond their traditional customer base.
Credit unions are not-for-profit organisations owned by their members.
The reforms will allow them to provide services to community groups, businesses and social enterprises.
Until now, their reach has been limited by a "common bond" that members must share - usually living or working in the same area, working for the same employer or belonging to the same association, such as a church or trade union.
They are now able to open up membership to new groups, such as housing association tenants or employees of a national company - even if some of them live outside a union's geographical area.
The new rules will also allow the organisations to pay interest on savings, instead of dividends, for the first time.
Robert Kelly, general manager of the NHS Credit Union for Scotland and the North of England, said: "The LRO is going to give us the opportunity to get closer to offering full services that are equitable and can compete with mainstream financial institutions like banks and building societies.
"It also gives us more potential for partnerships with a wide range of other organisations."
The Association of British Credit Unions (Abcul) expects membership to increase as a result of the changes, which the UK government introduced as part of a commitment to promote mutuals.
There are currently about 420 credit unions across England, Scotland and Wales, with close to one million members.
They have a strong presence in Scotland, particularly in the Glasgow area, where one in five people are estimated by Abcul to be members.