The number of companies convicted of offences following the deaths of construction workers has fallen sharply, a report suggests.
Research was conducted for construction union UCATT
But the number of workers killed in construction accidents last year rose by 25%, the study adds.
The report by building union UCATT shows prosecutions for deaths fell from 42% to 11% between 1998 and 2004.
However, the union's figures have been disputed by the Health and Safety Executive (HSE).
The study has been published to mark Workers Memorial Day on 28 April.
It often takes more than three years following the death of a construction worker before a company is brought to trial and convicted.
The report states that 504 construction workers died over a six year period to 2004.
It claimed there were huge regional differences in the likelihood of a company being prosecuted and there were disturbing trends in the level of fines.
The construction workers union has campaigned for a crack down on companies that jeopardise safety on building sites.
Alan Ritchie, leader of UCATT, said the failure of the HSE to prosecute was "profoundly shocking".
In a statement, the HSE said that in the majority of cases inspectors took appropriate action.
However, it said it recognised that further improvements were needed.
Geoffrey Podger, chief executive of the HSE, told the BBC that prosecutions are examined on a case-by-case basis.
"We don't have targets for prosecution," he said.
"Every prosecution has to be considered on the circumstances of the case - is the evidence available? Does it support a prosecution? Is it in the public interest?
"And we're like any other prosecutor, we've got to behave fairly. We can't just take the view there's been a ghastly incident so let's go off immediately and prosecute someone.
"And actually our overall rate of prosecution is going up, it's not going down, and we are, as far as we're concerned, a regulator with teeth and we're quite prepared to use them in this sector."