Neighbours have reported 130 people for watering their lawns since a hosepipe ban was imposed in Oxfordshire.
The ban on thousands of households from watering gardens or washing cars was imposed by Thames Water in April to cope with 18 months of low rainfall.
Since then, the company has warned 130 householders that they have been spotted flouting the regulations.
The water company is operating a 'three strikes and out' system.
First-time offenders are written to, a repeat offence wll incur another warning letter and a personal visit, while a third offence could mean prosecution.
Breach of a hosepipe ban is an offence under section 76 of the Water Industry Act 1991 and the maximum fine is £1,000.
Thames Water spokesman Nick Tennant said: "We always send them a letter first, but in 99.9 per cent of cases we haven't had to follow it up with a second letter."
The Thames region suffered 18 months of below average rainfall until 96mm of rain fell last month, making it the wettest May since 1983.
The above-average rainfall meant extra water could be taken from the region's rivers and diverted into reservoirs.
Thames Water is continuing to review the option of imposing a drought order which increases limits on the use of water on a weekly basis.
To report breaches of the rules, call the Thames Water customer service telephone line on 0845 9200 800.
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