Health bosses have warned that hospitals faced one of the "busiest days ever" this week as the heatwave continues around the country.
The weather is forecast to remain hot across the UK into next week and the NHS is experiencing near-record levels of demand at A&E departments.
The Met Office has forecast temperatures in the high 20s, with some areas reaching into the 30s.
Dan Stroud, spokesman for the Met Office, said conditions would become "increasingly humid and uncomfortable" over the weekend, especially in inner cities, with temperatures unlikely to drop below the 20s overnight.
NHS national medical director Professor Sir Stephen Powis said: "We know the warm weather has increased demand on services and this week we have seen one of our busiest days ever at A&E departments.
"As we move into what's forecasted to be a very warm weekend, the heat and the impact of industrial action will continue across the country.
"NHS staff are working hard and prioritising urgent and emergency care, so as ever, use 999 in emergency and life-threatening situations and NHS 111 online for other health concerns."
A UK Health Security Agency spokeswoman warned that the sustained heat could impact the health and social care sector.
She said: "If current forecasted temperatures are reached it is likely that there could be some impacts across the health and social care sector.
"A yellow alert means that any impacts include the increased use of health care services by vulnerable populations and an increase in risk to health for individuals over the age of 65 or those with pre-existing health conditions, including respiratory and cardiovascular diseases.
"If you have friends, family or neighbours who you know are more vulnerable to the effects of hot weather, it is important you check in on them and ensure they are aware of the forecasts and are following the necessary advice."
The Met Office said that most parts of the UK will see a continuation of the recent dry and sunny conditions, with temperatures peaking around 28C.
But from this afternoon, the weather is set to change across western parts of Northern Ireland at first, with thundery showers likely as well as some heavy downpours, with around 25mm of rain within an hour falling in some spots.
Showers may extend into parts of Wales and southwestern England, but most places will remain dry until the evening.
However, overnight and into Saturday there might be an increased risk of more showers and thunder, the Met Office said.
Met Office Deputy Chief Meteorologist Steven Keates said: "Over the coming days we'll be transitioning to a more unsettled regime for the UK, though temperatures will remain high and it'll feel very humid for many.
"Heavy showers and thunderstorms are likely to become more frequent through the weekend, with the potential for associated hail, lightning and some gusty winds. While the focus of thundery showers on Friday afternoon will be Northern Ireland, that risk spreads more widely across western and southern areas of the UK on Saturday, before pushing further north on Sunday.
"As in many of these situations, these showers can be hit or miss, with some places avoiding them whilst other areas nearby may see some very wet conditions.
"There's an ongoing likelihood of warnings being issued in the coming days, so keep an eye on the weather forecast for the latest outlook."
