Italian prosecutors on Saturday accused seven people and two sub-contractors of crimes including fraud and breaching airplane safety rules following an investigation into suspected flawed parts produced by an Italian company for Boeing.

The prosecutors launched their investigation in late 2021 after Boeing said some parts for its 787 Dreamliner plane supplied by a company working for Italian aerospace group Leonardo had been improperly manufactured.

Investigators found that two Italian sub-contractors used cheaper and non-compliant forms of titanium and aluminium to make certain parts, saving significant sums of money on their raw material costs, the prosecutors said in a statement, without naming the sub-contractors or the seven people.

"This resulted in the realisation of airplane parts with significantly lower static and stress resistance characteristics, with repercussions on aviation safety," the prosecutors in the southern city of Brindisi said.

Two sources familiar with the matter told Reuters that former Leonardo supplier Manufacturing Process Specification (MPS) and its now-bankrupt predecessor company Processi Speciali were the two firms at the centre of the probe.

MPS owner Antonio Ingrosso and his father Vincenzo, who headed Processi Speciali, were two of the seven people involved in the probe.

The two men are "convinced that they have acted respecting fully the law," their lawyer told Reuters.

The seven people and two sub-contractors will now be given time to present any new evidence in their defence, before the prosecutors decide whether to request a judge to call a trial.

Aerospace experts working with prosecutors certified at least 4,829 non-compliant components made of titanium and 1,158 made of aluminium, the prosecutors said.

"The expert work and investigations concluded that some non-compliant structural components could, in the long run, create harm to the safety of the aircraft, requiring the U.S. company to initiate an extraordinary maintenance campaign of the aircraft involved," they said, adding Boeing and Leonardo were victims of the alleged crimes and had cooperated with the probe.

Leonardo and Boeing declined to comment.