* FTSEurofirst down 0.3 pct
* Investors spooked by prospect of September rate rise
* Amadeus falls 6.3 pct as Air France mulls ticket change
By Liisa Tuhkanen and Alistair Smout
LONDON, June 9 (Reuters) - European stocks fell for the sixth consecutive session on Tuesday, led lower by Spain's Amadeus and tracking a fall on the U.S. equity market as investors speculated that the Federal Reserve may raise rates as soon as September.
The FTSEurofirst 300 was down 0.3 percent at 1,524.64 points by 0813 GMT after hitting a one-month low in early trade.
It tracked Wall Street, where the Dow Jones dropped into negative territory for the year on Monday.
Investors have grown more nervous about the timing of the Fed's first rate hike in nearly a decade following stronger-than-expected May jobs data released last week.
"We're agnostic as to whether the rate rise will be September or November... the balance of probabilities suggests it might be September," James Butterfill, global equity strategist at Coutts, said.
"So the markets are getting concerned about whether it will be correctly timed... (However) tightening monetary policy causes a bit of a wobble in equities, but it usually recovers," he added.
Spanish travel IT firm Amadeus was the top faller on the pan-European FTSEurofirst 300, dropping 6.3 percent to 34.36 euros.
The fall came as Air France-KLM said it is considering following a move by Lufthansa to levy a charge on tickets booked via third parties on global distribution systems (GDS) as a way of increasing its per-ticket earnings.
This drew the ire of Amadeus, which will now have to use a different system to avoid the charge, and the stock is down nearly 15 percent in June.
Broker Kepler Chevreux also cut its target price on Amadeus to 39.20 euros from 43 euros, saying Lufthansa's move added uncertainties to Amadeus' outlook.
"This new strategy raises concerns about the whole business model of GDS companies ... Lufthansa's move could be used as a marketing tool by other companies, which will be keeping a close eye on how things pan out," analysts at the brokerage said in a note.
"Although we agree that the market is probably discounting a negative scenario, we are reluctant to become more positive despite the price correction. We believe the uncertainties are here to stay."
Investors also kept an eye on negotiations between Greece and its international creditors.
The European Commission has received a new proposal from Greece on Tuesday, the day after German Chancellor Angela Merkel warned that time was running out for a reform-for-aid deal to keep the country in the euro.
Editing by Andrew Heavens