Temperatures peaked at 90F (32C) in Northolt, west London, beating the weekend high of 89F.
Overnight on Sunday temperatures reached 75F, making Britain hotter at night than Istanbul was at midday.
The heatwave will last through the rest of this week and is expected to peak by tomorrow or Thursday, according to the Met Office, before heavy downpours bring some relief.
A Government level-3 “heatwave action” alert remains in place in most of England until Thursday morning.
Thermometers nudged past 86F in Scotland and Birmingham.
Met Office forecaster Emma Sharples said temperatures could nudge even higher before the heatwave breaks.
“We expect the heat to continue over the next few days,” she said.
“There will be another surge of heat on Wednesday and we could see a repeat of the start of the week with similar temperatures to Monday – or even warmer.
“The South is again looking at the low to mid-30Cs, although there will be some thundery downpours further north.
It will feel fresher on Friday and into the weekend, although temperatures will stay in the mid-20Cs.
But you will feel the contrast.”
MeteoGroup forecaster Callum Stewart said: “The highest temperature recorded [on Sunday night] was 24C at London City Airport. Temperatures eventually dropped around 4am but they remained in the high teens and low 20Cs. We can expect more warm and humid nights, as the very warm weather will remain until Friday.”
Passengers at Bristol Temple Meads faced delays yesterday after the heat reportedly made rail tracks buckle.
Trains were diverted along alternative tracks.
But passenger Lauren Pinkett tweeted: “Temple Meads standstill since 2pm!!! HELP!”§