The Conservatives are to draft in a deployment of Republican activists to bolster their efforts in key seats before the UK election.

In an unusual move, a team organised by the US Young Republicans International Committee will arrive on 2 May to help out in the marginal seat of Enfield North, and the constituency of Aylesbury, where the Conservatives face a challenge from the UK Independence party (Ukip), as well as the safer seat of Windsor. The move will be controversial because they are set to campaign though they are not able to vote in the UK.

The Americans will arrive during a campaign in which the Conservatives lack ground troops in tight battleground seats compared with Labour, meaning they are relying on mailshots of election literature and busloads of mostly young Tory activists at weekends known as Team 2015.

This ​feels​ very much like a subversion of democracy Fiona Dent, Labour party candidate for Windsor

The expected arrival of the US activists has infuriated Labour candidates in the targeted seats, who pointed out the Republican party’s opposition to President Barack Obama’s healthcare reforms aimed at improving care for the poorest in society.

An invitation to attend the week urges Young Republicans International members to “help our Conservative colleagues win”. In an explanation of the electoral landscape, it adds: “Conservatives currently have the most support nationwide, but a clear majority in the Commons isn’t guaranteed. With strong opposition from Labour on the left and smaller parties splitting the vote, polling suggests this will be the closest-run election in decades. The identity of the next government and prime minister are far from certain.

“YRNF’s programme will begin on Saturday 2 May in London, followed by four days of campaigning for Conservative candidates in three different constituencies and a chance to participate in election day itself.”

A post about the UK election campaign on the Young Republicans International Facebook page. Photograph: Facebook

The Young Republicans are a movement for those between 18 and 40 across the US, supporting the rightwing Republican party. A document detailing the programme logistics of the trip, including the estimated $2,500 (£1,675) cost, appears to have disappeared or been removed from the group’s website since the Guardian made inquiries about the purpose of the trip and how many activists would be attending.

However, a page on the Young Republicans’ international website says the tour has been jointly organised by the group “and by our colleagues in the Conservative party”.

The Tories were also reluctant to answer questions about the deployment, ignoring two calls and two emails to ask whether it was appropriate for Young Republicans to be campaigning for them.

But Joan Ryan, Labour candidate for Enfield North and the area’s former MP, said: “The Conservatives’ betrayal on our local NHS is the key issue for many people in Enfield at this election. It says a lot when the Tories are now shipping in Republicans to salvage their campaign – a party whose entire political agenda is focused on stopping any form of public healthcare.”

Fiona Dent, the Labour candidate in Windsor, added: “It feels very much like a subversion of democracy when people who don’t even live in the country are brought in to campaign for [the Conservative candidate] in Windsor.”