Carrie Johnson has warned mothers who breastfeed to make sure they eat and drink enough in the summer heat, after telling her social media followers that she was admitted to hospital for severe dehydration.

The wife of former prime minister Boris Johnson shared a selfie from a hospital bed while cradling the couple's fourth child, five-week-old Poppy, on Friday evening.

Mrs Johnson, 37, wrote in an Instagram post alongside the photo: "Being hospitalised for two nights for severe dehydration was not on my postpartum bingo card."

She urged "breastfeeding mums" to make sure they eat and drink enough, "especially if your babe is clusterfeeding".

Mrs Johnson, who gave birth to the little girl - full name Poppy Eliza Josephine Johnson - on 21 May, thanked the NHS in a second Instagram post for looking after the pair during a "brutal" week.

She wrote: "This week has honestly been brutal.

"Mastitis (me), reflux (her), dehydration (me). What a pair we are!

"But thank you for all the kind messages, especially all the brilliant advice on reflux. Really appreciate it and made me feel way less alone going thru it all.

"And as ever, thanks to our amazing NHS."

The NHS recommends drinking more water when there's a higher risk of dehydrating, including when sweating due to hot weather.

Its website lists symptoms as feeling thirsty, having dark yellow, strong-smelling urine; urinating less often than usual; feeling dizzy or lightheaded; feeling tired; a dry mouth, lips and tongue.

In its breastfeeding advice, it says to drink plenty of fluids, including keeping a drink close when settling down to breastfeed, with water, lower fat milks, lower sugar or sugar-free drinks all recommended.

Heat health alerts came into force on Saturday across most of England as the country braces for a second summer heatwave.

An amber alert covers the East Midlands, south-east, south-west, east and London - meaning various health services and the whole population could be affected by the heat, according to the UK Health Security Agency (UKHSA).

Temperatures are forecast to rise into the 30s over the weekend, while Monday could be the hottest day of the year so far with a temperature of 34C, possibly 35C in London towards the Cambridgeshire area, according to BBC Weather.

The National Pharmacy Association, which represents about 6,000 community pharmacies in the UK, has warned people to keep medicines below 25C during the heatwave, unless the medicine has different storage instructions such as refrigeration.

Sukhi Basra, its vice-chair, said: "It's important we take sensible precautions, including double checking that your medication is stored correctly in a cool dry place as per any storage instructions, ideally below 25 degrees, as well drinking plenty of fluids and keeping your homes cool in the hottest hours of the day."