UPDATE 1-RBS Q3 loss 1.5 bln stg as bad debts hurt

LONDON, Nov 6 (Reuters) - Royal Bank of Scotland (RBS.L) made an operating loss of 1.5 billion pounds ($2.5 billion) in the third quarter as it took 3.3 billion pounds of bad debts and profits at its investment bank arm more than halved from the previous quarter.

Part-nationalised RBS said impairment losses were down 30 percent from 4.7 billion in the second quarter and were showing signs of levelling off, but were expected to "remain at elevated levels for the next few quarters".

RBS said its operating loss more than halved from 3.5 billion pounds in the second quarter. It reported a statutory loss of 2.2 billion pounds for the third quarter.

RBS, which agreed to massive asset sales and a revised participation in a state-backed asset insurance plan on Tuesday that will see the UK holding rise to 84 percent, said operating profit for its global banking and markets business was 375 million pounds in the latest quarter, down from 1.1 billion in the second quarter.

(Reporting by Clara Ferreira-Marques and Steve Slater)