The Met Office says the hot spell is set to be the first time that an area in the UK has reached over 30C for five consecutive days in June for more than 20 years.
Chief meteorologist Dan Suri said: "High pressure is dominating our weather, bringing warm, humid air from the tropical Atlantic resulting in these high temperatures and sunny conditions over the coming days."
Monday's temperatures touched 32C (90F), marginally higher than Sunday's, with Tuesday and Wednesday also expected to be scorching.
Public Health England has warned that older people, young children and those with underlying health conditions should take extra care in the heat.
The TUC has also urged employers to be more flexible with dress codes at work.
Overnight, the temperature reached 24C (75F) at London City Airport - hotter than Istanbul, where day temperatures did not climb higher than 21C (70F) on Sunday.
The warm weather is expected to stick around for the rest of the week, slowly climbing down to the mid to low 20s towards the weekend.(c) Sky News 2017: UK enjoys longest June hot spell for more than 20 years