A massive fire at a partially built Colindale development last year is likely to have been caused by workmen smoking at the site, an official report has concluded.

The Fire Protection Association (FPA), the UK's fire safety watchdog, also said it was more by luck than good judgement that no one was killed or seriously injured' in the blaze, which started at about 3.30pm, on July 12, and raged for five hours.

The fire, at Beaufort Park development, in Aerodrome Road, resulted in black smoke rising hundreds of feet into the air. It had a significant impact on the local community', noted the FPA's report, citing the massive smoke plume, traffic gridlock, and the evacuation of buildings and houses nearby.

Two thousand people living close to the development had to be evacuated from their homes, and adjacent Middlesex University halls of residence were evacuated after they too caught fire, causing £2million worth of damage. Nearby Colindale police station and police training college also had to be evacuated. Thirty cars parked nearby were badly damaged.

Block B4, where the fire started, collapsed nine minutes after the alarm was raised. Despite the efforts of 100 firefighters, the flames quickly spread to four other blocks. All were built on timber frames.

The report addressing the cause of the fire at the development, which is now secure and being rebuilt, rules out arson. However, while noting the site was no-smoking, there was evidence builders had broken that rule.

The findings also stated: "Although no formal report has yet been produced by the fire brigade, careless disposal of smoking materials, possibly igniting sawdust or wood shavings, appears to be the most likely cause."

The report also raises doubts as to the suitability of timber construction for high-rise buildings and highlights a lack of suitable escape routes' for workers, adding this is clearly unacceptable'.

Barnet councillor Brian Coleman, Conservative leader on the London Fire and Emergency Planning Authority (LFEPA), which runs London Fire Brigade, said: "I have always had serious concerns about these types of developments and the rapid fire spread at Beaufort Park convinced me that there was a very major problem.

"Having now had the opportunity to examine, in detail, the full report produced after considerable investigation and research, my fears have been confirmed and I shall therefore take every opportunity to campaign for an end to timber-framed construction.

"Apart from anything else there has to be consideration for the means of escape for construction workers."

Bryan Woodley, chief executive of UK Timber Frame Association, refuted Mr Coleman's comments, saying: "Timber frame and all other forms of mainstream construction meet the Government's regulations on fire, flood and other health hazards - end of story."

The 25-acre Beaufort Park development is a combination of housing, businesses and retail outlets. St George PLC, the company responsible for the five blocks ravaged by the fire, declined to comment.