Attempts by the Ministry of Defence to dismiss a leaked report highlighting increased risks from exposure to depleted uranium in shells backfired spectacularly yesterday when it emerged that not only was it written by an experienced military officer but it was endorsed by senior officers.

With defence ministers coming under renewed pressure to say what they knew of the health risks of depleted uranium (DU), officials first tried to discredit the report as the work of a "trainee".

But the MoD admitted last night that the report had been written by an "experienced officer". It added: "He was new to the post, with no experience of that particular area".

The report was then given more credence by a second internal MoD document. It emerged yesterday that the report - stressing long-term health risks from DU contamination - was attached to a covering letter from the office of the army's quartermaster general recommending its distribution to military and civilian personnel likely to come into contact with the armour-piercing shells.

The letter was signed by a senior retired officer on behalf of the quartermaster general's chief of staff. Dated April 1997, it warns that on impact "toxic and radioactive dust can be spread inside and outside of the [target] vehicle".

A further army document, dated August 1999, warns soldiers not to "enter or climb a damaged hard target or loiter within 50 metres", adding: "do not eat, drink, or smoke near the damaged vehicle.

"When an AFV [armoured vehicle] is penetrated by a DU round, the core becomes molten and may spread radioactive particles in the air."

In a letter to Geoffrey Hoon, the defence secretary, his shadow minister, Iain Duncan Smith, demanded to know if ministers were advised of the concerns about DU-tipped shells or told that the warnings were wrong.

Menzies Campbell, the Liberal Democrat defence spokesman, said: "The government's efforts to explain away documents relating to depleted uranium lacks credibility."

Faced with a growing problem of credibility, the MoD yesterday promised to publish the leaked documents with what it called a "suitable commentary" as soon as possible.

"Whilst accurate in the main, they contain some significant errors of scientific fact," it said. It referred to the warning in the 1997 document that uranium dust had been shown to increase the risks of developing lung, lymph and brain cancers. "It has not," the MoD said.

Its chief scientific adviser, Sir Keith O'Nions, said the report contained "many, many scientific errors" and did not form any part of the advice given to ministers.

Mr Hoon told Channel 4 News last night that he had not seen the document before it was leaked. "That document is not a document that was passed down the chain of command." He added: "What we are saying is that the risks are very small and have not led in any case that we have been able to establish by the best scientific evidence to any illness for any soldier."

John Spellar, the junior defence minister, infuriated Gulf war veterans earlier this week by announcing voluntary screening for Balkans veterans, without referring to them. Yet some 900,000 DU-tipped shells were fired in the Gulf, most by US aircraft, compared with 40,000 in the Balkans.

The Guardian has found that defence ministers claimed in 1993 that the shells did not produce "soluble depleted uranium". The MoD now says the risk is more from soluble DU than insoluble radiated dust.

The UN yesterday stepped up pressure for a survey of the areas hit by DU-tipped shells in Bosnia - and raised the prospect of a similar mission to Iraq - after traces of radioactivity and pieces of DU were found during a preliminary assessment of sites in Kosovo.