By Robert Harvey and Luiza Ilie

LONDON/BUCHAREST, Nov 11 (Reuters) - Oslo-listed Vantage Drilling VDI.OL cancelled a contract to drill next year, which two industry sources told Reuters on Wednesday was with sanctioned Russian oil major Lukoil LKHO.MM for exploration at its Romanian Black Sea Trident gas discovery.

The cancellation is the latest sign that Lukoil's overseas energy empire is buckling under the weight of U.S. and British sanctions, coming just days after it declared force majeure at its Iraqi oil field - the company's most prized foreign asset.

Vantage Drilling initially announced the cancellation of a 260-day contract for its drillship Platinum Explorer on October 19 without disclosing the location or client. Britain had imposed sanctions on Lukoil and fellow Russian oil company Rosneft ROSN.MM three days earlier.

Vantage Drilling declined to comment directly on the two sources' assertion that the contract had been with Lukoil.

"Vantage terminated the contract because the applicable sanctions made its performance unlawful," it told Reuters.

Lukoil did not respond to a request for comment.

The Russian company holds an 85% interest in the Trident and Est Rapsodia blocks alongside Romanian state-owned gas producer Romgaz. Following a six-year hiatus it had restarted procedures to resume exploratory drilling in 2026.

Romgaz SNG.BX did not respond to Reuters' request for comment.

Trident, which Lukoil said in 2015 could contain at least 30 billion cubic meters of gas, is among the high-profile discoveries in the Black Sea that energy majors including Shell and OMV are trying to tap in hopes of replacing lost Russian gas supplies with domestic EU production.

(Reporting by Robert Harvey in London, Nerijus Adomaitis in Oslo, America Hernandez in Paris, and Luiza Ilie in Bucharest; Editing by Joe Bavier)

((Robert.Harvey@thomsonreuters.com;))