An internal classified report drawn up for senior US defence officials concludes that the Serbs had access to Nato's daily orders for air raids and reconnaissance flights during the first two weeks of the allied bombing campaign, which began on the night of March 24.

By the end of the second week of the campaign, according to the report, Nato started to change the way the orders about bombing raids were distributed. The effect on what the Serbs appeared to know about Nato's bombing plans was immediate, it concludes.

The existence of the report is revealed in a BBC2 programme, Moral Combat: Nato at War, to be broadcast on Sunday night.

It says that the leaking of secret Nato war plans meant that the Serbs also knew when Nato's "detection assets" - spy planes and unmanned reconnaissance drones - would be deployed. Serb army and police units could move their men and equipment around with less risk of detection and in advance of bombing raids.

The report - an investigation into the lessons learned from the Kosovo conflict, which ran from March to June last year - says Belgrade's secret source was human and was not the result of hacking into Nato's coded computer system, called Chronos.

General Wesley Clark, Nato's supreme commander, suspected early in the bombing campaign that Belgrade had a spy in the organisation's Brussels headquarters, according to a senior source there. "I know I've got a spy, I want to find him," he is said to have told colleagues.

Nato military chiefs immediately checked the distribution of the daily "air tasking orders" or ATOs. To their horror, they discovered that 600 people had access to them. "You could have knocked me down with a feather", said one senior US air force officer who asked not to be identified.

The large distribution list - understood to comprise military personnel and unlikely to have included diplomats or members of Nato governments - was based on the number of people authorised to see operational details of the alliance's earlier no-fly zones over Bosnia. The Bosnia plans had been agreed with the Serbs, so were not sensitive.

The top-secret Nato air orders were an entirely different matter. As soon as US commanders knew how many people were on the distribution list, they narrowed it to 100.

Another source, in the Pentagon - the US defence department - also said there was evidence of a Serb spy. Pilots at Nato's operational headquarters at Vicenza in northern Italy were expressing concern that the Serbs were "picking up on our runs".

Speaking on the record to the BBC, Gen Clark was asked if he thought there was a spy "at Nato headquarters". He replied: "Absolutely not". When asked whether there was a spy "in Nato", he said: "I don't think so." He confirmed, however, that there was an investigation into Nato's procedures. "We restricted who had access to the air tasking orders and we tightened them up and we kept some sensitive items off the ATOs," he said.

He added: "I think they [the Serbs] knew about the categories we were going after on the first night."

It was reported last year that the US F117A Stealth fighter shot down by a Serb anti-aircraft missile in the first week of the bombing campaign may have been attacked after its secret flight plan was passed to Belgrade by a spy at Nato headquarters.

A Nato official was quoted last August as saying that an alliance military officer had passed details of the Stealth mission to a Russian intelligence official who tipped off Belgrade.

In November 1998, a French officer, Pierre-Henri Bunel, admitted that the month before he had passed on Nato's overall target plans to a Yugoslav diplomat in Brussels. This was in October last year, five months before the bombing began. He was reported as saying that he had acted out of hatred for the US.

Of Nato's 19 member countries, two - Italy and Greece - maintained diplomatic missions in Belgrade during the conflict. The secret US report is understood to give no indication of how a Nato spy could have passed information to Belgrade. Nor does it speculate about his nationality.

The BBC programme also reveals that a Swedish financier was sent on a secret mission to Moscow and Belgrade during the war. He told President Slobodan Milosevic that the Russians had agreed to a peace plan saying: "We cannot help you ... so exit now."

Moral Combat: Nato at War will be broadcast by BBC2 on Sunday. Allan Little is foreign affairs correspondent for the BBC