Space tourism trips to the International Space Station will no longer be possible from May 2009 when the station increases its crew compliment from three to six, European Space Agency ISS programme sources have told Flight.

US company Space Adventures, which has co-operated with Russia's Federal Space Agency (FSA) to provide places for the tourists - called spaceflight participants - will no longer be able to offer flights.

Since 2001 private citizens have visited the ISS on 10-day trips accommodated in the spare seat on the FSA's Energia Soyuz-TMA flights. But the crew increase on ISS to six means that all the available Soyuz seats will be occupied.

However Space Adventures insists it can still provide flight opportunities: "We have secured the April 2009 seat. We are working to secure additional seats for 2010, and beyond."

The next and sixth tourist will be computer game entrepreneur Richard Garriott, son of former NASA astronaut Owen Garriott, who started training for his October flight last month.

The first space tourist to the ISS was US citizen Dennis Tito whose trip was in 2001. Since then there have been four other tourists including Iranian-American business woman Anousha Ansari, whose family sponsored the $10 million suborbital X Prize, and former Microsoft software developer Charles Simonyi.