The co-pilot of EgyptAir Flight 990, which crashed off New England in 1999, killing 217 people, deliberately crashed the plane as an act of revenge, according to a source close to the investigation.

Gamil el-Batouty had earlier been reprimanded for sexual misconduct and the executive who told him he would not be allowed to fly US routes again was on board the plane.

The report by the National Transportation Safety Board is due to be published shortly. It will conclude that el-Batouty forced the New York-to-Cairo plane down. The report will not attribute motives to him as to why he took the action nor will it suggest that he deliberately crashed the flight.

However, sources close to the investigation paint a picture of el-Batouty as a man facing ruin in the light of a series of allegations of sexual misconduct, including exposing himself to teenage girls, propositioning hotel maids and stalking female hotel guests.

On board the doomed flight was Hatem Rushdy, the chief of EgyptAir's Boeing 767 pilot group, who had just reprimanded el-Batouty. He told him that as a result of his sexual activities he would not fly transatlantic routes - which carry extra pay - again.

"Rushdy told him 'this is your last flight' and el-Batouty's attitude was 'this is the last flight for you too'," the former EgyptAir captain Hanofy Taha Mahmoud Hamdy told the Los Angeles Times.

A high-ranking member of the US team agreed. He said: "It was more revenge against Rushdy than just a suicide."

The blackbox recordings indicate that Rushdy had left his first-class seat and entered the cockpit. But as soon as he left, el-Batouty, who was supposed to fly the second five-hour section of the flight, entered the cockpit and ordered a younger first officer out of his chair.

Saying "I rely on God", he switched off the auto-pilot and pushed the plane into a dive.

Captain Ahmed el-Habashi, who had been out of the cockpit managed to struggle back in, and shouted: "What's happening, Gamil? What's happening?" El-Batouty said again "I rely on God," and shut down the fuel to both engines.

Habishi shouted: "What is this? Did you shut the engines?" and grabbed the controls. "Pull with me! Pull with me!" he screamed.

The plane, which had been in a nosedive, then soared back up to 24,000 feet, but lost all power and broke up from the stress.

"I think [el-Batouty] had been looking at the door, looking at the moment when Rushdy is out of the cockpit," Mr Hamdy said. "This is the most suitable moment to fulfil what he is going to do."

The claims are likely to exacerbate already tense American-Egyptian diplomatic ties.

The US has been heavily criticised in Cairo for exploring the possibility that el-Batouty had deliberately crashed the plane.

A top Egyptian investigator said yesterday his country would reject any suggestion that el-Batouty was to blame.