(Corrects currency conversion in fourth para, trillion not billion)
By Steve Slater
LONDON, April 15 (Reuters) - Britain's top banks, grappling with how to split future "ring-fenced" UK retail arms from riskier investment banking operations, face a multi-trillion-dollar dilemma: what goes in, what stays out?
Under proposals from a UK panel looking at ways to shield savers and taxpayers from a repeat of the global financial crisis, banks must set up separate UK retail subsidiaries.
The sheer size of Britain's top banks, whose balance sheets dwarf its economy, and the global nature of the likes of HSBC and Barclays, whose operations include extensive investment banking operations, makes it a daunting prospect.
The UK retail arms could cover over 2.5 trillion pounds ($4.1 trillion) of assets, or maybe just a third of that, depending on what is included.
But many details about the structure of those units and what will go on to their books will only be clarified in six months time.
BASIC SERVICES
Basic retail deposit taking will be included, and investment banking services for big corporates will not.
But what happens to businesses in between, such as payment services, unsecured loans or dealings with small businesses, is unclear.
The final design will have a key bearing on how much it will cost banks, how big a task the split will be and the scale of the assets the UK taxpayer will be standing behind.
The proposal will force the likes of Barclays, Royal Bank of Scotland and HSBC to hold more capital for retail unit assets.
It could also increase funding costs and squeeze profits.
But the outcome should be less severe than having to wall off their investment banks or being required to undergo a more formal split.
"The Commission has come up with something that is pragmatic," said Mike Trippitt, analyst at Oriel Securities.
"The essence of the report is adequate capitalisation and some sort of segregation or ring-fencing ... anything else would have been costly, massively disruptive and potentially unworkable."
The Independent Commission on Banking (ICB) estimated banks would have to garage a minimum of 15 percent of assets with the retail unit, leaving 55 percent of assets outside the ring-fence, notably capital markets services and activities by big corporates. The remaining 30 percent could also be included.
"The devil is in the detail on how you ring-fence the retail bank, as it's not clear," Trippitt said.
The top five lenders could have to park 2.6 trillion pounds ($4,228 billion) of assets, almost twice the size of the UK economy, based on their combined balance sheet of 5.75 trillion pounds.
But under a more restrictive definition of what needs to be shielded, the amount could be less than 900 billion pounds.
The ICB acknowledged its report left many questions unanswered. Whether it will cover credit cards, wealth management, riskier retail mortgages and most areas of business banking remains uncertain.
INTERACTION
How the subsidiary can interact with other operations will influence funding and be key to the cost of the plan, bankers and analysts said.
The cost will be well below the 12 billion pounds some industry estimates have suggested, the ICB said.
ICB Chairman John Vickers said the implications of the extra costs would likely be reduced profitability, lower remuneration for bankers and increased costs for customers.
The ICB wants to make the ring-fence strong enough to shield depositors if trouble flares up in other areas, and make it easier to wind down the retail business if needed. But it wants banks to benefit from some efficiency gains.
"Ideally, it would be possible for the rest of the group to support the retail entity but not to jeopardise it, and vice versa," the ICB's report said. (Editing by David Cowell)