Labour is sending almost 100 current and former party staff members to the US to help Democrats in swing states.
Activists from Sir Keir Starmer's party will spend almost three weeks campaigning for Kamala Harris ahead of the US election on November 5.
They will visit Nevada, North Carolina, Pennsylvania and Virginia - states that are crucial to Ms Harris' bid to win.
However, Republicans have described the plans as an “outrage” and warned they would damage the UK’s relationship with the US should Donald Trump win the presidency.
The Federal Election Commission, which regulates US elections, states that foreign volunteers may not donate money to campaigns or spend money on their behalf.
They cannot “participate in the decision-making process” but they may, however, participate in activities as an “uncompensated volunteer”.
Elon Musk reacted to the news on X, formerly Twitter, by saying: “This is illegal.”