UPDATE 1-Spot platinum falls 3 pct to 18-month low

LONDON, Sept 10 (Reuters) - Platinum prices fell by more than 3 percent to an 18-month low of $1,196.50 an ounce on Wednesday on increasing worries about demand from automakers, traders said.

Spot platinum XPT= was at $1,200.50/1,218.50 an ounce at 0845 GMT from $1,236.00/1,256.50 an ounce late in New York on Tuesday.

The metal has lost almost 50 percent of its value since hitting an all-time high of $2,290 an ounce in March this year.

"The dominant use for platinum group metals in the real world is in autocatalysts," said Stephen Briggs, commodity strategist at RBS Global Banking and Markets.

"Car sales are proving pretty disastrous at the moment, so that is what is driving them down."

(Reporting by Pratima Desai; additional reporting by Jan Harvey; editing by Christopher Johnson)