(Adds background, Vitol spokeswoman's comment, details)
By Ron Bousso and Jessica Donati
LONDON, March 13 (Reuters) - Vitol, the world's largest oil trading house, has been barred by pricing agency Platts from its daily European "window" naphtha trading session after failing to deliver on time some of the oil product that it had sold, traders said.
A ban from the window - a 30 minute session where market participants conduct trades that are visible to all - does not prevent a company from trading outside it but significantly reduces a trader's clout in the market.
A spokeswoman for Vitol said on Wednesday that just one naphtha cargo had been delayed, without commenting further.
Platts said it would not comment on specific names of companies it deals with.
"From time to time we do review the participation of companies in our assessment processes," Platts said.
"We never confirm the names of the companies under review. But you can see the list of the participants in the window each day," said Dave Ernsberger, Platts Global Editorial Director.
The Swiss trading house, which assumed a pivotal role over the past year in Europe's physical market for naphtha - an oil product used for blending into gasoline or to make plastics - abruptly disappeared from the Platts window after Feb. 21.
It has since been absent.
According to several naphtha traders who do business with Vitol, Platts suspended Vitol from trading in the window around two weeks ago, after it failed to deliver some cargoes on time.
Trader sources said it was unclear how long the ban would remain in place.
Two traders said the number of delayed cargoes was near 15. None of the traders said that their own companies had been directly affected by the cargo delays.
The delays had led buyers to complain to Vitol and in some cases to suspend further trading with it, according to traders.
Vitol had revenues of more than $300 billion in 2012 and naphtha represents a small portion of its trading portfolio.
The company continues to trade crude oil and other products in the Platts windows.
Vitol became the most active buyer on the naphtha market in 2012 before switching to selling in the second half of January.
Its buying and then selling helped to influence prices of naphtha, traders said.
It sold around half of the 30 naphtha cargoes traded during the window between Feb. 1 and Feb. 21, and a higher volume in January.
Naphtha cargoes traded during the Platts window are typically around 12,500 tonnes. Vitol sold a cargo on Feb. 21 at $995 a tonne.
At around the time of its big selling campaign in Europe, Vitol bought a cargo of around 330,000 barrels shipped by a European refiner to the New York region and then, in a highly unusual move, turned it back to Europe.
One trader said this was done to meet earlier commitments. (Additional reporting by Simon Falush, Peg Mackey, Claire Milhench, Julia Payne in London, Jessica Jaganathan in Singapore; Editing by Anthony Barker)