The current situation has put an incredible stress on the men and women of Nasa

Daniel Goldin, Nasa

But Russian space officials later denied making such an agreement.

Mr Tito, a 60-year-old multimillionaire, paid Russia some $20m (�14m) to travel to the International Space Station (ISS).

According to his son, Mr Tito is spending his time in space looking out of the window, listening to music and getting his money's worth.

After initial objections, Nasa eventually agreed to allow Mr Tito on board the ISS, provided he did not enter the US modules unescorted.

Guidelines

Formal guidelines are being drawn up in a bid to avoid a repeat of the embarrassing disagreement between Nasa and its Russian partners over whether Mr Tito should have been allowed on board.

"The current situation has put an incredible stress on the men and women of Nasa," Mr Goldin told a US House of Representatives panel on space and aeronautics on Wednesday.

"Mr Tito does not realise the efforts of thousands of people, in the United States and Russia, who are working to protect his safety and the safety of everyone else."

The Nasa chief said the cost of Mr Tito's voyage - in money and lost research time - would be assessed after the mission ends and that Russia had agreed to reimburse these costs.

Russian space officials later denied making such an agreement.

"There is no agreement" on such a reimbursement, Russian Space Agency spokesman Sergei Gorbunov told the news agency AFP on Thursday.

He added that the agency was "in the dark" about the matter.

Titanic

Daniel Goldin also praised another potential amateur astronaut - film director James Cameron - for delaying any trip until the time was right.

He said the director and producer of the film Titanic had approached him about six months ago and asked him about going into space.

When Mr Goldin told Mr Cameron there was no protocol for screening or training, or even a known time when he might fly, Mr Goldin said the director had told him he would delay any trip until an "appropriate time".

Mr Tito arrived at the station on Monday. He will stay for about six days, then return to Earth aboard a Soyuz craft.