* Court says exchange rate spread on fx loan was unfair, invalid
* Latest turn in legal saga on big pile of forex mortgages
* Government has said would phase out fx mortgages
* Court says could make key broad ruling on June 16
By Krisztina Than
BUDAPEST, June 3 (Reuters) - Hungary's top court ruled on Tuesday that the exchange rate margin applied by OTP Bank on a foreign currency mortgage was unfair and invalid, a ruling that could guide government plans to tackle problem forex loans.
Prime Minister Viktor Orban, elected for a second term in April, is planning a new relief scheme for households whose Swiss franc and euro loans have become expensive to service.
Banks fear the programme will inflict new losses on the sector after years of heavy taxation and earlier tough government measures to help troubled borrowers.
Orban's government has said it will await a final ruling by the top national court, called the Kuria, before it decides how to deal with about $15 billion worth of forex mortgages mostly taken out by households before the 2008 financial crisis.
The court said that ruling could be made on June 16.
Tuesday's decision already provides some clues to what may be in store for the country's mostly foreign-owned banks.
Tuesday's case was brought by a Hungarian borrower who complained that he got a Swiss franc loan from OTP
The Kuria said this exchange rate spread - the difference between the rate at which the loan was disbursed and at which installments are paid - was unfair and invalid.
OTP shares fell after the ruling and were down 2 percent at 4,780 forints by 1007 GMT.
The Kuria said OTP would have to apply the National Bank of Hungary's official exchange rate to both the disbursement amount and the repayments.
Akos Kuti, an analyst at brokerage Equilor, said that based on market estimates and his calculations, compensating borrowers for the exchange rate spread could cost the financial sector up to 50 billion forints in a worst-case scenario.
Another analyst, Andras Balatoni at ING, said: "The profitability of OTP will decline by HUF 5 bln, while the effect on the whole banking sector can amount to HUF20-40 billion."
Kuti said the final ruling by the top court on another aspect of the loans - whether banks were sufficiently transparent about unilateral changes such as interest rate hikes - was more important and could be more costly for the banks.
"As for OTP and the banks, today's is not the decisive ruling, but the one which will affect the issue of contract modifications (interest rate changes)," Kuti said.
Low interest rates made foreign currency loans, mainly in the safe-haven Swiss franc, popular in Hungary before the 2008 financial crisis, but they turned sour as the Hungarian forint weakened, making the loans much more expensive to repay.
Many borrowers were pushed into default.
At 1009 GMT, the forint
Hungary's largely foreign-owned banks have already lost more than a billion euros in a previous, 2011 relief scheme.
They include units of Belgium's KBC
(Reporting by Krisztina Than; Editing by Catherine Evans)
((krisztina.than@thomsonreuters.com)(+36 1 327 4745)(Reuters Messaging: krisztina.than.thomsonreuters.com@reuters.net))
Keywords: HUNGARY COURT/LOANS
