A DRAFT report on the Clutha helicopter disaster has been circulated to lawyers acting for victims and other interested parties.

The confidential dossier was finally distributed in April. It is the last step in the long-running investigation into the fatal crash, with sources close to the inquiry saying the final report could be made public next month.

The draft version outlines the conclusions reached by investigators at the Air Accident Investigation Branch (AAIB), who were tasked with pouring through evidence from the wreckage, software systems and eyewitness reports to determine the likely cause of the tragedy.

Ten people were killed when a Eurocopter EC135, operated by Bond on behalf of Police Scotland, crashed through the roof of the Clutha Vaults pub in Glasgow on the night of November 29 2013. It had been returning from a routine operation when, according to eyewitnesses, it "dropped like a stone".

The content of the draft report is top secret, but it is distributed to all the key legal representatives to give them the opportunity to query or challenge any of the conclusions reached by investigators ahead of a final version being produced.

Challenges by legal teams can potentially delay the publication of a final report, which will be pivotal in the decision of the Crown Office to launch a subsequent prosecution or Fatal Accident Inquiry.

Lawyers acting for victims' families, Police Scotland, Bond, and the aircraft manufacturer, Airbus Helicopter, Eurocopter, will all have received a copy.

A source close to one legal team said: "A draft report has been circulated. Of course, the draft might be quite different from the final report, but we are preparing on the basis of the final report being ready in June."

Air crash investigation is reserved to Westminster through the AAIB, a division of the Department for Transport.

In February, it emerged that First Minister Nicola Sturgeon had written to the Prime Minister criticising the length of time the probe was taking and the "dearth of information" being passed to police and prosecutors north of the Border.

The last interim bulletin on the tragedy was published in February 2014, and the AAIB said in November last year that it expected to circulate the draft report in "early 2015" with a final report in "mid-2015".

Lengthy investigations are not unusual, however.

The final report on the Super Puma helicopter crash, which claimed the lives of 16 offshore workers in April 2009, took 18 months to complete.

A final report is also still pending into the fatal helicopter crash near Shetland in August 2013, in which four oil rig employees died.

However, the frustration around the Clutha investigation has been exacerbated by the questions raised by the most recent interim report, which left industry experts baffled.

Investigators revealed that the police helicopter had crashed after suffering double engine failure caused by fuel starvation - even though there was still 76kg of fuel on board at the moment of impact.

It emerged that the fuel had not been reaching the engines because key fuel transfer pumps had been switched off, probably at some point during the flight.

How this situation arose was the key mystery facing investigators, complicated by the absence of black box flight recorders.

A spokesman for Bond said it would be "inappropriate to comment on the report before it has been published".

A spokesman for the AAIB said: "The draft report has been circulated to 'interested parties', as defined by the AAIB's regulations, and the final report is due in the middle of 2015."