A patient waited 62 hours for an ambulance, while four trusts took more than 24 hours to respond to 999 calls, new figures have shown.
The longest delays in the UK were recorded by Welsh Ambulance Service, which kept four patients waiting for more than 50 hours.
A spokesman said the figures were "not typical" and "represent the extreme end of the waiting time spectrum".
The Patients Association said they were "extremely concerning".
Between June 2017 and June 2018, ambulances from four services took 24 hours to reach patients, including some with breathing and mental health problems.