By Tom Freke

LONDON (Reuters) - Billionaire Simon Halabi has been asked to repay a 1.15 billion pound debt, which may force the sale of some of his trophy London offices, a source close to the situation said on Thursday.

Bondholders met on Wednesday to decide the fate of the debt, which is backed by a portfolio of nine London office buildings, following a default earlier in July.

Their decision was to "accelerate" the bonds -- which form the securitisation vehicle White Tower 2006-3 -- meaning the debts "have become immediately due and payable", according to a statement from bond administrators Hatfield Philips.

Companies controlled by Simon Halabi, ranked Britain's 14th richest person by the Sunday Times in 2007, have a short period to repay the debts or secure an alternative agreement with Hatfield Philips, the source said.

Nobody at Halabi's Buckingham Securities returned Reuters' requests for comment.

If an agreement cannot be reached or the debt repaid, a receiver could be appointed to start the process of selling the buildings, which include JP Morgan's (JPM.N: Quote, Profile, Research) Alban Gate offices and a landmark City of London building occupied by insurer Aviva Plc (AV.L: Quote, Profile, Research), the source said.

The default in the debt occurred after a revaluation of the nine buildings cut their value to 929 million pounds, about half their worth when the bonds were sold in 2006.

The repayment demand may also be a mechanism to preserve income for bondholders from lower-ranked "B" debtors, owed 285 million pounds, the source added.