Germany's energy regulator has suspended the approval process for the controversial Nord Stream 2 natural gas pipeline from Russia to Germany.
It said the pipeline's operating company needed to be compliant with German law before it would certify the €10bn (£8.4bn) project.
The decision sent UK and mainland Europe wholesale gas prices, already under pressure, to three-week highs.
Critics fear the pipeline will increase Europe's energy dependence on Russia.
Russia's state-owned Gazprom said the pipeline was ready in September, but it has been beset by delays.
Running under the Baltic Sea, Nord Stream 2 will double Moscow's gas exports to Germany, but it will also circumvent Ukraine, which relies on existing pipelines for income and would be hard-hit by the loss of transit fees.
German businesses have invested heavily in the 1,225km (760-mile) pipeline and former Chancellor Gerhard Schröder has played a big role in its development.
