HEALTH and Safety Executive off icials have been threatening to prosecute builders' merchants unless they hand over lists of what their customers are buying.
Stunned merchants received letters last month demanding information on contractors buying dense concrete blocks weighing more than 20 kg.
The HSE warned that refusing the request would be viewed as a criminal offence.
Health officials are cracking down on the use of heavy blocks as part of their Better Backs Campaign.
But merchants have attacked their latest approach as 'heavy handed' and have forced the HSE to withdraw its threats.
Builders' Merchants Federation secretary Peter Matthews said: 'It was quite a shock for our members to get these letters and it has completely backfired on them because it damages people's belief in what they are there to do. It is almost an abuse of power and a very heavy-handed approach.'
The HSE has now apologised for causing concern and advised its regional offices to withdraw any mention of possible criminal sanctions from letters to merchants.
Mr Matthews said: 'When we raised the issue they immediately saw it was a problem and were very good about rectifying it.
'I think they wanted to show they are taking this whole campaign very seriously but it looks like they were trying to get tough with the wrong people.'
One merchant said: 'I couldn't believe it when I got the letter. There's no way I'm going to break customer confidentiality and I don't like being threatened like this.'
An HSE spokesman said: 'We have been pursuing a successful campaign against the unwarranted supply and use of heavy blocks in the building industry for some years.
'Inspectors have powers under section 20 of the HSW Act 1974 to require the provision of information. But the HSE is very happy to work with BMF to obtain this information on a voluntary basis, so we can target those who continue to use these blocks unnecessarily.'