The move is expected to impact family-size chocolate bars and share bags sold in the hospital stores of chains such as WH Smith and Marks & Spencer.
Hospital chiefs must guarantee four out of five items purchased on their premises do not exceed the 250 calorie limit.
In 2018/19, there will also be financial incentives for health services to ensure 80% of confectionery on site does not exceed 250 kcal.
NHS: ‘super-size’ snack culture causing obesity epidemic
Simon Stevens, NHS England’s chief executive, said: “The NHS is now stepping up action to combat the ‘super-size’ snack culture, which is causing an epidemic of obesity, preventable diabetes, tooth decay, heart disease and cancer.
“In place of calorie-laden, sugary snacks, we want to make healthier food an easy option for hospital staff, patients and visitors.”
NHS has already removed ads, price promotions and stopped checkout sales on sugary drinks and foods high in fat, sugar or salt. This includes confectionery and sweets over 250 kcal.
NHS’ latest rules on confectionery align with its guidelines known as CQUIN (Commissioning for Quality and Innovation), which covers food and drink sold to patients visitors and staff.
NHS premises in England serve around 1m patients a day, as well as staff.
Almost 53% of NHS staff are overweight or obese, estimates the authority.
The Royal Voluntary Service is among the largest NHS food & drink retailer with 440 cafés, shops and trolley services. Other chains include WH Smith, Marks & Spencer, Greggs, Starbucks, Subway, Costa Coffee, Medirest and ISS.
In 2014, Food and Drink Federation (FDF) members, including Mondelēz International, signed up to a 250 calorie cap on single-serve confectionery sold in the UK.
However, larger bags and bars are not considered single-serve but for sharing.
Public Health England (PHE) recommended chocolate should contain 43.7g of sugar per 100g by 2020 in voluntary health guidelines published earlier this year in a bid to cut sugar by at least 20% by 2020.
Chocolate confectionery in the UK currently has an average portion size of 44 g and contains an average sugar content of 54.6 g per 100 g with 197 calories per portion, according to PHE.
An FDF spokesperson told ConfectioneryNews: "FDF members have already voluntarily committed to a 250 kcal cap on individually wrapped, single-serve confectionery and ice-cream items sold in retail, which has resulted in calorie reductions of between 10% and 15% in many top-selling branded products.
"FDF and our members are continuing to work with PHE on its ambitious Sugars Reduction Programme whilst at the same time engaging fully with the newer focus on calories.”