The share of that age group among all players rose from 8.7 to 13.5 per cent in the two years from September 2019.
Some 624,377 extra people over 65 gambled online at least once a month in that period.
A sharp rise of 341,445 gamblers in the 45-54 age group was the second greatest increase.
But in the 16-34 age group, the number who gambled online fell by around 307,000.
The Royal College of Psychiatrists (RCP), which gathered the data, says always available online gambling carries more risks at the moment as people are at home more and may face financial difficulties.
Prof Henrietta Bowden-Jones warned that gambling can lead to "depression, anxiety and suicidal thoughts".
The Royal College of Psychiatrists (RCP), which gathered the data, says always available online gambling carries more risks at the moment as people are at home more and may face financial difficulties.
Prof Henrietta Bowden-Jones warned that gambling can lead to "depression, anxiety and suicidal thoughts".