The Swiss sun-powered aircraft Solar Impulse will attempt to fly from Switzerland to Morocco in coming weeks, in its longest flight to date, organisers said on Wednesday.

"After its inaugural flight to Paris and Brussels in 2011, Bertrand Piccard and Andre Borschberg's solar airplane will attempt, for the first time ever, to fly over 2,500 km without using a drop of fuel, finally landing in Morocco," said the project in a statement.

Piccard and Borschberg will take turns to pilot the plane during the 48-hour journey, with a stopover near Madrid expected for the swap.

The trip, expected for May or June, would be a rehearsal in the run-up to the plane's round-the-world flight planned for 2014.

"Flying as far as this, powered only by solar energy will be excellent training for the round-the-world trip," said Borschberg, co-founder and chief executive of Solar Impulse.

The high-tech aircraft, which has the wingspan of a large airliner but weighs no more than a saloon car, made history in July 2010 as the first manned plane to fly around the clock on the sun's energy.

It holds a record for the longest flight by a manned solar-powered aeroplane after staying aloft for 26 hours, 10 minutes and 19 seconds above Switzerland, also setting a record for altitude by flying at 9,235 metres.

It has since flown several times, notably between the Geneva and Zurich airports, as well as to Paris and Brussels.