An inquiry into interference by hostile foreign states in the UK should be extended to cover the actions of Donald Trump’s US, the Liberal Democrats have said.

In a letter to the communities secretary, Steve Reed, whose department is leading on the independent review, the Lib Dems said the US government’s explicit support for far-right nationalist parties in Europe amounted to outside interference.

The US national security strategy, set out this month, said Europe faces “civilisational erasure” due to migration and EU integration, and that Washington should “cultivate resistance” within the continent.

The document used language echoing the racist “great replacement” conspiracy theory, saying several countries risked becoming “majority non-European”, and praised the “growing influence of patriotic European parties”.

The inquiry into the effect of financial influence and other interference is expected to primarily focus on Russia and similar hostile states. It was announced on Tuesday after the jailing of Nathan Gill, a former senior figure in Reform UK.

In the letter to Reed, Calum Miller, the Lib Dems’ foreign affairs spokesperson, Lisa Smart, the party’s Cabinet Office spokesperson, and Zöe Franklin, who shadows Reed, praised the inquiry and said foreign interference in the UK was “a real and growing threat”.

It read: “Though the impetus for this investigation follows grave concerns about the extent of Russia’s penetration of our political system, we must be clear-eyed too about the threat of interference now posed by the United States.

“In his recent national security strategy, President Trump outlined the US’s new policy of ‘cultivating resistance’ within Europe. This reflects nothing less than an explicit call for interference in European politics – including our own.

“Will the government commit, therefore, to including the US administration – including President Trump and his associates – in the scope of its investigation? Anything less would constitute an egregious blindspot in our efforts to ensure British politics is free from outside manipulation.”

The Lib Dems have repeatedly sought to push Keir Starmer’s government to take a more robust stance against a US president who, polling shows, is unpopular with many UK voters.

In September, during Trump’s second state visit to the UK Ed Davey, the Lib Dem leader, boycotted a state banquet hosted by King Charles in honour of the US president as a protest against what Davey said was a failure to intervene decisively to end the war in Gaza.

In a separate statement, Miller said: “Donald Trump has made it his explicit policy to interfere in European politics and ‘cultivate resistance’ in the UK and elsewhere.

“Failure to include Trump’s government as part of this new probe would be deeply irresponsible and would leave a gaping hole in our defences against the manipulation of our politics.”