Ambulances were turned away from the A&E department at Royal Preston Hospital after it became too busy.
Ambulance crews reported that they were diverted to Blackburn between 7.30pm and 8.30pm on Thursday night after the A&E became too full to cope.
The divert, which is the first of the winter season in Preston, comes as national NHS bosses warn of seriously over-stretched services.
Many hospitals are reportedly running at 99 per cent of capacity already, although the safe limit is 85 per cent. Medics have warned that pressures are likely to become more severe as the weather worsens and winter flu hits.
Lancashire Teaching Hospitals Trust declined to comment.