UPDATE 1-PKN suffers $341 million Q1 loss, as expected

* Swings to net loss of $341 mln

* Loss due to financial costs, revaluations

* Margins deteriorate (Adds operating details, background)

WARSAW, May 14 (Reuters) - Poland's largest refiner, PKN Orlen PKNA.WA, had a 1.1 billion zlotys ($341 million) net loss, roughly in line with expectations, as costs grew with a weaker Polish currency and lower oil prices reduced the value of stored reserves.

Analysts in a Reuters poll had expected PKN to report a net loss of 1.2 billion zlotys. The refiner posted a 626 million zloty net profit in the year-ago period.

Lower refining margins, inventory revaluations and a smaller difference between the price of Ural crude oil and Brent oil, pushed the refiner's operating result down by 858 million zlotys in the quarter, it said.

A weakening zloty inflated the refiner's costs on its euro-denominated loans and forced it to charge 842 million in foreign exchange losses.

PKN's revenue dropped 18 percent year-on-year to 14 billion zlotys.

PKN shares have risen 8.8 percent this year, outperforming a 4 percent gain of Warsaw's main WIG20 index .WIG20. (Reporting by Patryk Wasilewski; Editing by Richard Chang)