Nasa does not yet know where it will get the money, but the space agency has added another shuttle launch to its schedule – the final one for the fleet.

The agency set a target launch date of June 28 for shuttle Atlantis and started preparations for the 135th and last shuttle flight.

The four-member crew will take up supplies to the International Space Station, make one spacewalk, and return a faulty pump that has bedevilled engineers.

Now three missions remain before Nasa retires its shuttle fleet this year. Shuttle Discovery’s last mission is planned for February 24, Endeavour’s in April.

The decision allows different parts of the shuttle programme to start work on Atlantis’ 12-day flight, including astronaut training and mission planning, Nasa spokesman Michael Curie said.

Originally Atlantis was planned as an emergency-only rescue mission if needed for the Endeavour crew.

Last year, the Obama administration and the US Congress clashed over the future of the human space programme and came up with a compromise that authorised one extra shuttle flight – the Atlantis mission. But congress never gave Nasa the few hundred million dollars needed for the extra flight, leaving the agency in a quandary about whether the flight was real or not.

The initial money is coming from the space shuttle programme’s regular budget, but that is not the big amounts needed for a shuttle flight, Mr Curie says.

“We’re optimistic that the funding will be there,” he said, but he could not give details about where the money would come from.

Nasa was pressed to start preparations or the Atlantis mission would not have been able to launch in late June, Mr Curie said.

The final flight will be commanded by Christopher Ferguson and includes Douglas Hurley, Sandra Magnus and Rex Walheim.

The extra flight means that Mark Kelly – the husband of Arizona congresswoman Gabrielle Giffords, shot in an attempted assassination – will not command the final shuttle flight even if he stays on the Endeavour mission.

With his wife’s expected long rehabilitation, Mr Kelly asked for a back-up commander to be named in case he could not fly as scheduled in April.