During March just 84.6% of patients were seen in four hours - well below the 95% target.
The previous worst month was February and before that it was December in what has proved to be an extremely difficult winter for the health service.
The publication of the March data by NHS England also means the picture for the full 2017-18 year is now available.
That shows annual performance is also the worst on record since the target was introduced in 2004.
During 2017-18, 88.4% of patients were seen in four hours.
The full year of data is not available yet for the other UK nations.
During winter, performance in Scotland has been similar to that in England.
Wales and Northern Ireland have seen much worse waiting times.
Neither of those countries have ever hit the four-hour target.