Labour has written to the elections watchdog urging pre-emptive action to ensure a US-based pro-Brexit lobbying group, co-run by Nigel Farage, does not risk “dark money” from overseas subverting the UK electoral system.

Farage, leader of the Brexit party, hosted a launch for the new group, called World4Brexit, or W4B, in New York last week, alongside American political figures who support Donald Trump.

The W4B website, which has a page soliciting donations of between $100 and $1,000, says the group was formed “by friends of the UK from around the world who welcome it back to the international community free of the EU”.

It quotes people including, Donald Trump Jr, and the former Australian prime minister Tony Abbott, but does not give details about who is behind the organisation. It says only that it is “committed to the ideals of free and fair elections, upholding the results of those elections and the essential power of democracy”.

In a letter to the Electoral Commission, shadow cabinet minister Chris Matheson said the watchdog should examine W4B’s sources of funding, what use the money was put to, and any potential links with the Brexit party.

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Matheson wrote to Bob Posner, the commission’s chief executive, saying: “We are in exceptional times, with exceptional individual and organisations, whose governance and interaction with each other is often secretive and whose respect for electoral law often appears absent.”

The letter does not allege wrongdoing by W4B or Farage but stresses that as a US-registered social welfare, non-profit, organisation W4B is not obliged to disclose its funders except in limited circumstances.

“It appears that money raised anonymously in the US and possibly other countries will find its way to the UK,” the letter says. “This in itself is not illegal, but it does raise urgent questions as to what that money will be used for, and what steps the Electoral Commission should take to reassure the public that no electoral laws are broken.”

Under electoral law “non-party” campaigners can spend such overseas funds outside election periods. However, if the money is spent directly on candidates or parties this breaches rules, as party donors have to be registered in the UK as an individual or organisation.

Ahead of the launch Farage told the Financial Times that all donations would be “above the board and legal”, with the cash used not for candidates but to “dig deep, find out who is really running the show”.

In his letter, Matheson said that while those reassurances were welcome, vigilance was needed, given a report last month by the commission saying the Brexit party was at high risk of receiving and accepting illegal donations due to its fundraising structure.

Matheson said the commission should seek to ensure any W4B money from overseas did not get used for UK candidates or parties. It should also examine any potential links with the Brexit party, and ascertain whether W4B could count as a “members’ association” – a group that is not a political party but mainly composed of members.

He wrote: “Democracy, here in the UK and in the US and Europe, has in recent years been subverted by vast amounts of dark money and by individuals and organisations intent on disregarding electoral law to the benefit of extreme rightwing ideas.” This had, he added, “undermined public trust in democracy”.

An Electoral Commission spokeswoman said the organisation had received the letter and would respond “in due course”.

Farage’s representatives were contacted for comment.