Lloyds and Royal Bank of Scotland customers face up to four years of uncertainty because of the sweeping overhaul of Britain's banking sector.
Banks that have benefited from state aid are being forced to sell some of their branches and customer base to ensure they do not have an advantage over rivals that managed without taxpayer money.
The requirements, set out by the government last week, followed the announcement that Northern Rock is to be broken up into a "good" and a "bad" bank, with the former being sold to a private buyer and the latter kept in state ownership.
Lloyds Banking Group, which is 43.5%-owned by the taxpayer and includes Halifax, Bank of Scotland, Intelligent Finance (IF) and Cheltenham & Gloucester (C&G), has to sell savings and current accounts worth £30 billion and £70 billion of mortgages. It must cut its current account market share by 4.6 percentage points from the present figure of about 28%.
Royal Bank of Scotland (RBS), which is 70%-owned by the taxpayer and includes NatWest and insurance firms such as Direct Line and Churchill, will have to shed 318 branches, about 14% of its total network.
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The consumer group Which? estimates that 3.5m to 4m current accounts will be shed by Lloyds and about 600,000 mortgages. RBS will lose about 1.7m bank customers, including current accounts and mortgages, and 17m insurance customers.
The government also confirmed a further £39.2 billion of taxpayer support for RBS and Lloyds. In exchange, the banks renewed their commitment to the lending targets set by the government for the two years ending March 2011 - £25 billion a year from RBS and £14 billion annually from Lloyds.
Here we answer your questions about the great banking shake-up:
What is being sold?
Over the next four years, Lloyds and RBS will shed hundreds of branches and millions of customers to rival firms.
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About 20% of the Lloyds network will be sold, including 185 branches of Lloyds TSB in Scotland, 250 Lloyds TSB branches in England and Wales, and 164 C&G branches. Customers of Lloyds' private banking services will not be affected.
IF, the online and telephone bank previously owned by Halifax, will go.
RBS will lose 311 branches in England and Wales and seven NatWest branches in Scotland. No NatWest branches in England and Wales will go. RBS will also have to lose the Direct Line, Churchill, Privilege and Green Flag insurance firms.
What if I didn't sign up through a branch?
You will have been assigned a local branch when you took out your current account, even if you joined over the phone or online. Your account is linked to that branch by the sort code and will be sold on that basis.
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How will Lloyds decide which branches to sell?
It is not yet clear. Lloyds said: "No particular group or type of customer will be targeted. Our remaining customer base will be representative of the group's current customer base."
Will I have a choice?
Yes, you will be able to stay with Lloyds or RBS if you wish but you will have to open a new account with an unsold branch to get a new sort code.
A Treasury spokesman said: "Customers will be entitled to open a new account with their old provider free of charge and with no fuss. We expect that customers' account numbers and direct debts will remain unchanged. Benefit payments, tax credits and wages will all be paid into accounts in the normal way."
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If too many people stay with Lloyds and RBS, more branches may need to be sold so that the banks can meet their targets for reducing market share.
What about my C&G account?
All branch-based C&G savings and mortgage customers will be sold on but customers who signed up for a mortgage over the phone will stay with the Lloyds group.
Those who took out their C&G mortgages through a broker more than about five years ago and had their accounts processed through a branch will also be kept. Lloyds will hold on to the C&G name, which will probably be used to sell mortgages through brokers. However, savings may again be offered at a later date.
C&G savers who do not want to move will have to open a Lloyds savings account instead and will use Lloyds branches.
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Our deal may just vanish
Mark Donnan, 38, a business development manager from Carmel, north Wales and his wife, Susan, 32, a web designer, signed up to a three-year tracker with Intelligent Finance last year using the comparison firm moneysupermarket.com.
It charges 1.04% (Bank rate plus 0.54%) before reverting to the provider's standard variable rate of 2.5% (Bank rate plus 2%).
Though the couple do not plan to move, they are worried that the IF brand will be sold. Mark said: "We have no idea who the new buyers will be or what will happen to our deal when it comes to an end.
"One of the reasons we signed up to IF was because of its association with what we thought was a big and respected brand, HBOS. If it goes to a much smaller company, I would consider switching."
Jo Howe, 34, and her husband James, 34, often use their local Lloyds branch. Jo said: "I rely on its services. Though I use internet banking, I also like to use the branch."