An agreement signed Tuesday between Russia's Space agency and a United States private spaceflight company confirmed space tourists will return to the International Space Station (ISS) before the end of 2021.

To date, the ISS has been visited by seven private citizens as paying customers of the US company Space Adventures, flying to the ISS on board a Russian Soyuz spacecraft.

The official Space Adventure's website issued the following statement: "State Space Corporation Roscosmos and Space Adventures, Inc. have signed a contract for the implementation of the short duration space flights of two spaceflight participants on board the same Soyuz spacecraft to the Russian segment of the International Space Station."

It said the flight was scheduled in late 2021.

Roscosmos has been a Space Adventure Space tourism partner since 2001 when it transported Dennis Tito, the first space tourist, to the ISS and back.

Founded in 1998, Space Adventures is the world’s first private spaceflight company and the only one currently providing opportunities for private astronauts to fly to and live in space.

Their seven client's on eight spaceflights (one customer flew twice), spent over 80 days in space altogether, clocking nearly 48.3 million kilometers (30 million miles) in space.

Space Adventures’ aims to open spaceflight and the space frontier to private citizens, and seeks to develop new spaceflight experiences in an exciting, affordable yet safe, manner.

The ISS project has resulted in two decades of international co-operation between 16 countries (United States, Russia, Japan, Canada, and major European countries). It has cost over $100bn (88,63bn euros) to build and operate.

A total of 230 individuals from 18 nations have now visited the ISS.

Only seven of these were neither cosmonauts nor astronauts but rather wealthy individuals such as Hungarian multi-millionaire Charles Simonyi, who decided to travel twice to the ISS.

The last private individual to spend a few tens of millions of dollars to take the ultimate "power ride" up to the orbital station was Canadian clown Guy LaLiberté in 2009.

According to the Energia, the Russian corporation that builds the Soyuz spaceships, the decision to suspend space tourism flights was taken some years ago after the US retired its winged shuttle fleet and Russia's spacecraft became the only remaining link between Earth's surface and the ISS to carry out essential crew rotations and resupplies.