The UK foreign secretary, Boris Johnson, has joined the US in condemning “divisive” German plans to press ahead with the construction of the Nord Stream 2 gas pipeline from Russia to Germany, arguing it could leave European energy consumers heavily dependent on “a malign Russian state”.

The giant pipeline, and the terms on which it is built, has become the litmus test of a dispute on how Europe can reach a working relationship with Moscow and yet defend its liberal values from a Russian threat.

In a letter released to pro-Polish British MPs, Johnson says it is right to highlight the “divisiveness of this pipeline across Europe”, adding that “Euro-Atlantic unity remains our strongest tool in standing up to malign Russian activity”.

He also says energy security must be driven by regulations that reduce the ability of any single supplier to exert monopoly control. He insists the EU is right to call for amendments to EU directives to ensure there is a level playing field between gas connectors, a change that would require Germany to allow non-Russian gas to use the pipelines.

Although Johnson in his letter does not specifically call for the pipeline to be halted, he says the UK stands firm in its support for Ukraine on the issue. Ukraine has been leading the opposition to the pipeline, as Russia plans to bypass the current pipeline routed through Ukraine that provides Kiev with badly needed annual income worth 2% of its GDP.

The 745-mile (1,200km) $11.7bn (£8.7bn) Nord Stream 2 is due to be completed in 2019 and will run through the Baltic Sea to Germany. It will follow the track of the existing Nord Stream 1, doubling the pipeline’s annual capacity to 110bn cubic metres. Germany says the pipeline is necessary to fill an energy gap left by the phasing out of its nuclear plants, and the projected decline in supply from Norway and the Netherlands, as their reserves wane.

In recent weeks Germany has come under intensifying American pressure to halt the pipeline, forcing Angela Merkel on to the defensive over her claim that the pipeline’s construction is strictly a commercial decision with no political implications. The issue is starting to dominate major bilateral discussions in Europe, including a summit last Friday in Sochi between the German chancellor and Vladimir Putin. Merkel has been urging the Russian president to guarantee the Ukraine pipeline will still be used, alongside Nord Stream, but he has said that will only happen if it is economically justified.

The US pressure stems in part from genuine concern at a Russian stranglehold on EU energy needs, but Germany has viewed the US involvement as a self-interested commercial intervention designed to increase its liquid gas sales to the European market.

The UK, preoccupied by Brexit, has until now tried to keep a low profile on Nord Stream even though its closest natural allies in Europe, the Baltic states, Poland, Hungary and the Ukraine, all fiercely oppose the pipeline on current terms.

Johnson will also be aware that backbench Tory opposition to the pipeline comes from Eurosceptic MPs wary of German influence inside Europe. Commentators in Russian newspapers such as Pravda this week confidently claimed that UK energy needs would become the anvil on which British opposition to Russia collapses. Reliant on the evaporating reserves of the Norwegians and Dutch for liquid gas, the UK will be forced to turn to Russian gas, and “beg Russia for forgiveness”, Pravda claimed.

There also fears among Euro-sceptics that the Foreign Office has been holding back from condemning Nord Stream because the UK government does not want to alienate Germany at a sensitive moment in the Brexit talks. Daniel Kawczynski, the chair of the all-party parliamentary group on Poland, said: “The letter is not as forthright as it could be, and in some way obfuscates, but importantly it accepts there are implications to the pipeline that extend well beyond what is in Germany’s economic self-interest.”

The final German permits for the construction of the pipeline, opposed by the European commission, were issued last month, ironically just days after Berlin symbolically expelled four Russian diplomats in protest at Moscow’s involvement in the poisoning of the former spy Sergei Skripal and his daughter, Yulia.

But the new pipeline is not a done deal, with opposition growing across Europe, including among MEPs. The US injected a fresh element into the discussion when Sandra Oudkirk, its deputy assistant secretary of state for energy diplomacy, said the US feared the pipeline would allow Moscow to place new listening and monitoring technology in the Baltic Sea.

She said Congress had given the US president new powers to impose sanctions against a variety of Russian pipeline projects.

Germany is looking for a deal that somehow keeps Kiev as a supplier, believing this could normalise relations between Ukraine, Russia and Germany. The EU’s objections would be overridden.