Your employer's duties are set out in the The Manual Handling Operations Regulations 1992. These Regulations require your employer to apply control measures to prevent, or reduce, the risk of injury to you from manual handling of loads.
The Regulations set out the approach your employer should take in three stages:
The Health and Safety Executive (HSE) also publishes advice on the sort of weights that are likely to cause injury in a risk assessment filter (see Appendix 3 of its guidance on the Regulations).


The HSE guidance says that the weights shown in the diagrams are not meant to be interpreted as 'safe limits'. You may still be injured lifting lighter loads if other 'risk factors' are present, eg an awkward lifting position, or if it exceeds your individual capability.
If you are handling the kinds of weights shown in the figures then a risk assessment is likely to be needed. The figures assume that you are lifting easily held, compact loads in ideal conditions.
The HSE has published Manual Handling Assessment Charts to identify ways of helping to prevent musculoskeletal disorders, available here.
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