Bosses say they are looking closely at staff attendance records, as a survey suggests many workers are prepared to fake illness during the hot spell.
Hot and humid weather will continue, the Met Office says.
Some 77% of employees are prepared to take a "sickie" and stay at home to enjoy the sunshine, according to employment law firm Peninsula's survey.
It questioned 700 workers and employers from around the UK.
Storms have disrupted the heatwave in many parts of the UK, but further hot weather is expected later in the week.
Peninsula managing director Peter Dome said businesses had indicated they were increasingly monitoring staff attendance records, amid fears that many people's recent days off have not been for genuine reasons.
The hot spell has prompted unions to call for employers to introduce practices to make working in the heat more bearable.
Last week, as temperatures reached record levels, the TUC said workers should be allowed to dress more casually.
It said it believed air conditioning could be turned down or even turned off if there was a more relaxed dress code.
Meanwhile, retail union Usdaw has lobbied the government for a change in the law to bring in a legally enforceable maximum working temperature.
Wednesday was the hottest July day on record in England, with a temperature of 36.5C (97.3F) recorded at Wisley, Surrey.
Forecasts suggest that temperatures in the mid-30s will be experienced again in many parts of the country on Wednesday and Thursday.
The Department of Health has advised people to stay out of the heat, particularly between 1100 BST and 1500 BST
They should also wear light, loose fitting clothes, it said.
And it said it was particularly important to check on older neighbours and relatives, who were more prone to the effects of the heatwave.