Euan Blair, the prime minister's eldest son, is to work in Washington DC as an intern for Republican politicians.
Euan Blair is about to graduate from Bristol University
The 21-year-old will spend three months working for the Committee on Rules in the House of Representatives - the lower chamber of the US Congress.
The move has surprised Democrats, who see the committee as highly partisan, says the Sunday Telegraph.
A Downing Street spokesman said the prime minister's son would also seek an internship with a Democrat politician.
Euan Blair is soon to graduate from Bristol University with an ancient history degree.
According to the committee's website, internships aim at giving students exposure to the political process.
"Duties our interns are responsible for include filing, assisting senior staff, sorting through mail, preparing for committee meetings and assembling folders," the website stated.
The Sunday Telegraph claimed Californian congressman David Dreier, chair of the committee, would be Euan's mentor.
Mr Dreier was a part of the team assembled by California governor Arnold Schwarzenegger to help the state tackle its economic crisis in 2003.
The newspaper reported the move had prompted surprise among Democrat politicians, who see the UK's Labour Party as their natural ally.
An aide to congresswoman Louise Slaughter, the leading Democrat on the committee, described it as "one of the most partisan" in Congress.
Eric Burns told the newspaper: "It is extremely surprising that the son of a Labour prime minister would intern with the Republican majority staff on the committee."
Downing Street said British diplomats had been involved in the process.