Astronomer Martin Rees
told CNN that the European Space Agency (ESA) should focus on robotic exploration instead.
"The practical case for sending people into space is getting weaker as robots are getting better," Rees said.
"Spaceflights have become an adventure sport," he explained. "The space station only makes the news when something goes wrong or when Chris Hadfield plays the guitar."
Rees said the mission of Tim Peake -- who became the first British astronaut to reach the International Space Station (ISS) last December -- was worth the cost. But the ISS has become more of a public interest venture than a scientific one, he said.
"The space station altogether has cost a twelve-figure sum because we've been up there for a long time and I don't think anyone could justify it to science since man space exploration has slowed down," Rees said.
The astronomer said human missions should be funded by private companies like SpaceX
or Blue Origin
"Each time a shuttle failure has happened it's held up the program for three years, which is being paid by taxpayers' money," he said. "Space flights should be left to adventurers prepared to accept higher risks."
About 365 million euros ($408 million) of the ESA's budget is spent on human spaceflight. Instead, Rees suggests all funding should be spent on unmanned missions so the agency can become "world leaders in robotics and miniaturization."
But ESA's director for human spaceflight and robotic exploration, David Parker, disagrees. In a statement to CNN he called Peake's mission the "largest science engagement project ever accomplished in the UK."
"ESA does believe that human spaceflight is excellent value for money," Parker said.
While costing each European citizen only about 1 euro ($1.12) per year -- and at around 7% of ESA's total budget -- human spaceflight "delivers excelling science and innovation," he said.
Peake, too, has said his mission has been "exceptionally worthwhile." He believes the UK will miss out if it is not at the forefront of space exploration.
Despite Rees' comments, he does hope to see a human mission carried out on Mars.
"I do hope that some pioneers will eventually land on Mars," he said. "But I think they'll either be Chinese or they'll be privately funded adventurers taking high risks and one-way tickets."