Tragedy struck on 8 September 2000 when Christopher Pullen went to play with friends in Southdown House, a badly vandalised block next to the one where he lived on Islington's Market Estate.
He died on 13 September 2000 when his life support machine was switched off, Mr Justice Stanley Burnton told the court.
Christopher's mother, Karen, challenged the Health and Safety Executive's decision not to prosecute the owners of the estate - Islington LBC - or the and Hyde Housing Association Ltd which was responsible for maintenance.
But she suffered further heartbreak today when Mr Justice Burnton dismissed her case, saying the HSE had rightly formed the view that there was 'no realistic prospect of a successful prosecution'.
He told the court: 'It was incumbent on the HSE to consider carefully the question whether to institute a prosecution following any death of a child in circumstances as tragic as the present case.
'It bears a heavy responsibility if it decides not to prosecute. But a decision to prosecute in such a case is equally weighty.'
Dismissing Mrs Pullen's judicial review challenge, he concluded: 'I am satisfied that the decision made by the HSE was one that was reasonably open to it. It follows that the claim must be dismissed.'
The judge said there was little direct evidence as to how the door came to fall on Christopher. The estate was blighted by vandalism and there was a high level of disrepair, but an inquest into the youngster's death returned an accidental death verdict.
It was 'regrettable', added the judge, that Mrs Pullen only learnt of the HSE's January 2003 decision not to prosecute from a newspaper article. Christopher's father, from whom she is separated, had been informed, but had not passed the information onto her.
It was the HSE's view that Islington and Hyde 'had a system for dealing with risks that was as effective as was reasonably practicable in the very difficult circumstances prevailing on the estate and that there was insufficient evidence of fault on the part of any of their employees.'
Mrs Pullen's case was believed to be the first ever judicial challenge to a decision by the Health and Safety Executive not to prosecute.
STRAND NEWS SERVICE