Nasa has begun to wind down construction of the rockets and spacecraft that were to have taken astronauts back to the Moon - effectively dismantling the US human spaceflight programme despite a congressional ban on its doing so.

Legislators have accused President Obama's Administration of contriving to slip the termination of the Constellation programme through the back door to avoid a battle on Capitol Hill.

Constellation aimed to build upon what was arguably America's greatest technological achievement, the first lunar landing of 1969, by launching new expeditions to the Moon and to Mars and worlds beyond. Mr Obama proposed in February that it should be scrapped because it was "over budget, behind schedule and lacking in innovation", but he has met opposition in Congress, which has yet to approve his plan.The head of Nasa, Major-General Charlie Bolden - an Obama appointee - has now written to aerospace contractors telling them to cut back immediately on Constellation-related projects costing almost $1 billion (£690 million), to comply with regulations requiring them to budget for possible contract termination costs.

The move has been branded a "disingenuous legal manoeuvre" and referred to Nasa's inspector-general for investigation. "It's bordering on arrogance by the Administration to boldly and brazenly go forward with this approach. It shows a blatant disregard for Congress," said the Republican Congressman Rob Bishop, of Utah, whose constituency stands to lose thousands of jobs. Two weeks ago the Senate passed legislation that compels Nasa to continue work on Constellation unless Congress directs otherwise. That legislation is due to be signed into law by Mr Obama this month while Congress continues its deliberations over his proposal to cancel the current space space progamme.

Senator Kay Bailey Hutchison, a Texas Republican and member of the Senate's Commerce, Science and Transportation Committee, said: "The timing of Nasa's decision to push forward with these actions now, before this becomes law, is highly questionable." Nasa is "willfully subverting the repeatedly expressed will of Congress", she added.

Scott Pace, a former Nasa executive and now the Director of the Space Policy Institute at George Washington University, said: "The effect will be to stop work on Constellation and lay off or transfer people to other jobs. If Congress then says it wants to continue going ahead with Constellation, those people will be difficult to re-hire. It's already a difficult situation, but this will introduce more instability."

Constellation was born in 2004 from President George W. Bush's vision for returning Americans to the Moon by 2020 and using it as a base to build the knowledge and technologies for a manned mission to Mars by 2030. Since then, more than $9 billion has been spent on designing and building the necessary space vehicles.

An independent review panel appointed by Mr Obama last year concluded, however, that without an extra $3 billion a year Constellation was on an "unsustainable trajectory". In his proposed budget for the 2011 fiscal year, unveiled in February, Mr Obama made it clear that there would be no extra money for its continuation. The proposal has yet to clear Congress.

Distinguished space veterans, including the first and last men to walk on the Moon, Neil Armstrong and Gene Cernan, have complained that the abandonment of Constellation will set America's space capabilities on a "downhill slide to mediocrity". They say that, while Mr Obama has outlined a vision for Nasa that includes sending people to Mars at some point, it lacks a concise plan for developing the rockets and spacecraft to get them there.

"The Administration has no planning, no programme and no idea - they'd just have these things happen mysteriously," Mr Bishop said. "Rockets aren't something that Wal-Mart puts on its shelves. You have to have a plan for how you get from A to B, and Obama has just said we'll work it as we go along and maybe some day we'll end up on an asteroid or the Moon or somewhere. The bottom line is, those 'maybes' will never happen."

Private rocket developers, to whom Mr Obama proposes outsourcing the task of carrying crews and cargo to the International Space Station after the shuttle fleet retires, are making advances. Ten days ago Elon Musk, a spaceflight entrepreneur - and founder of the online payment system PayPal - launched a near-flawless test flight of his Falcon 9 rocket, which is designed to take payloads and ultimately human beings into space.

Triumphs and tragedies

Oct 4, 1958 A year after the Soviet Union launches Sputnik, the US Congress passes "an Act to provide for research into the problems of flight within and outside the Earth's atmosphere, and for other purposes". The National Aeronautics and Space Administration (Nasa) is born

Feb 20, 1962 John Glenn becomes the first American to orbit Earth. The Russian Yuri Gagarin had made the first space flight a year earlier. Glenn returns to public adulation and later becomes a US senator

May 25, 1961 President Kennedy announces that he is setting the United States the goal of reaching the Moon by the end of the decade

January 27, 1967 Three US astronauts die in a fire during a simulated take-off, the first to die in the space programme

July 20, 1969 Neil Armstrong takes man's first step on the Moon and plants an American flag, a significant propaganda coup. The US is still the only nation to have put a man on the Moon

April 13, 1970 An oxygen tank explodes aboard Apollo 13. It becomes clear that there is not enough air in the capsule to keep the three astronauts alive. They manage to board the self-contained Lunar Module and land safely in the Pacific

April 12, 1981 The US launches Columbia, its first space shuttle and the first spacecraft to land on a runway instead of in the sea

Jan 28, 1986 The Challenger space shuttle explodes 73 seconds after take-off, killing its crew

April 24, 1990 The Hubble Space Telescope comes online after being carried into space by a US shuttle

Feb 1, 2003 Columbia breaks up over Texas returning from its 28th mission, killing the crew

Feb 1, 2010 President Obama announces plans to cancel additional funding of the programme to return US astronauts to the Moon by 2020