LONDON, Sept 13 (Reuters) - Chinese banks saw net capital outflows of $109 billion in the first quarter of 2015 and weak incentives to hold renmimbi positions indicate capital may keep trickling out, a BIS report said on Sunday.

In its quarterly review, the Bank for International Settlements said that international banking statistics showed the money had flowed from banks inside China to overseas banks, putting downward pressure on the currency.

"These results offer clues as to what may happen in the third quarter, during which China changed its exchange rate policy," BIS said

China devalued the yuan - or renmimbi - on Aug. 11 and has been spending heavily to hold the currency steady onshore as fears have grown that the economy is in worse shape than previously thought.

Central bank reserves declined by a record $94 billion in August and authorities this week instructed banks to bolster management of foreign exchange transactions and pay attention to suspicious transactions.

BIS said pressure on the yuan had been building since the first quarter as a cut in deposit rates and a slight depreciation in mid-March reduced incentives to hold the local currency.

"As a result, option-implied currency volatility rose. In short, the interest rate differential narrowed and the cost of insuring against further depreciation increased, reducing incentives for the long-renminbi position," BIS added.

It explained that companies wishing to reduce yuan exposure had likely boosted hard currency deposits at Chinese banks - indeed central bank data shows such deposits grew by a net $49 billion in the first quarter.

Meanwhile yuan-denominated deposits held by the rest of the world at Chinese banks fell, amounting to a net outflow of almost $109 billion, the report added.

While yuan deposits partially recovered in the second quarter, thanks to exchange rate stability, the BIS said incentives to hold yuan were reduced again in the third quarter as authorities cut deposit rates and implemented a devaluation.

It added there was evidence of some offshore renminbi deposits being liquidated.

"At the height of the recent equity and exchange market turbulence, the offshore renminbi interbank interest rates in Hong Kong reached the unprecedented levels of 10.1 percent for the one-week and 8.6 percent for the one-month tenor, reinforcing reports of renminbi selling," the report added.

(Reporting by Sujata Rao; Editing by Ruth Pitchford)