US Christian fundamentalist groups have spent over $50m (£38m) in Europe over the past decade, and have dramatically ramped up campaigning in recent years, according to a report by the UK political website OpenDemocracy.

The analysis comes amid growing concern about the extent to which western democracies can be influenced using “dark money”, political funding from unknown sources. None of the groups examined by OpenDemocracy identify their financial backers.

Foundations and charities in the US are required to file a publicly available Form 990 document annually. This document sets out details of an organisation’s spending for the financial year, including top-level figures for funds spent in Europe.

The Alliance Defending Freedom, which campaigned in favour of a law in Belize making gay sex punishable by 10 years’ imprisonment, has tripled its spending in Europe to £1.5m annually.

It spends £430,000 each year lobbying EU officials, according to separate EU transparency data. It told OpenDemocracy it was “exclusively privately funded by people from all over the world, who care about human rights”.

Another of the organisations is the Acton Institute for the Study of Religion and Liberty, which has spent over £1.3m in Europe since 2008.

The group has previously co-sponsored a conference in Italy with the Dignitatis Humanae Institute, a Catholic organisation supported by the far-right campaigner Steve Bannon with plans to transform a former monastery into a “gladiator school for culture warriors”.

Several of the organisations are partners of the World Congress of Families, which has been designated a hate group by the Southern Poverty Law Center (SPLC) for its campaigns against LGBT civil and human rights.

The WCF is organising a conference in Verona, Italy this weekend that is expected to be attended by several far-right European activists and politicians, including the Italian deputy prime minister Matteo Salvini, leader of the far-right party the League.

Others scheduled to attend include a Nigerian activist who has compared gay people to the Islamist terrorist group Boko Haram, and a Ugandan politician who campaigned for gay people to be jailed for life or executed.

The WCF told OpenDemocracy it disputed “entirely” the premise of its designation as a hate group by the SPLC.

A group of 40 members of the European parliament has written to the European Commission to express concern over US rightwing organisations’ growing efforts to cultivate influence in Europe.

The letter, signed by members of the green, socialist and liberal groups, called on the commission to examine the funding “with greatest urgency” ahead of the May European elections.

Caroline Lucas MP, the former leader of the Green party in the UK, told OpenDemocracy: “It’s deeply worrying to see US far-right millionaires exporting their particular brand of hate-filled politics to Europe.”

“If these groups are going to operate in Europe’s courts and in the European parliament, the EU should at the very least force them to reveal who is bankrolling them.”