Moscow, Mar 17 (IANS/RIA Novosti): Unesco Artist for Peace ambassador and the world's best-selling soprano singer Sarah Brightman's trip to the International Space Station (ISS) in 2015 is in doubt, the head of Russian space agency Roscosmos has said.

Roscosmos and NASA may opt against sending Brightman to the ISS because the trip depends on the duration of the 2015 visiting flight to the ISS, said Vladimir Popovkin.

The British singer was set to go on an eight-day trip to the ISS, but NASA and Roscosmos are considering extending the visiting flight to one month, in which case she would have to give up her seat to a professional spaceman, Popovkin said.

"If it's a monthly shift, then it will be staff cosmonauts and astronauts who will be performing some short-term scientific research," he said.

Popovkin did not say when the decision will be made or whether Brightman's flight could be rescheduled for a later date.

Neither did he name Brightman's potential replacement, saying only that the candidate will be fielded by the European Space Agency.

Brightman, 52, the world's best-selling soprano singer with 30 million CDs sold, announced her plans to go into space in August 2012.

In case she makes it to the ISS, the star would become the world's eighth space tourist.

The first was US entrepreneur Dennis Tito in 2011, and the last, so far, was Cirque du Soleil co-founder Guy Laliberte, who paid $40 million to spend 12 days at the ISS in 2009.

Two ISS crew members who would arrive to the ISS in 2015 will stay in orbit for a year, up from a mission's current standard duration of six months.

This would not be enough, however, to beat the current record of longest uninterrupted stay in orbit, held by Russian Valery Polyakov who spend 439 days in space in 1994-1995.