Patients' lives are being put at risk because of delays giving them treatment for sepsis, experts are warning.
Hospitals are meant to put patients on an antibiotic drip within an hour when sepsis is suspected - but research by BBC News suggests a quarter of patients in England wait longer.
Delays raise the chance of potentially fatal complications such as organ failure.
But NHS England said there were signs performance was improving.
And it said hospitals were getting better at spotting those at risk sooner.
Dr Ron Daniels, of the UK Sepsis Trust, said the "concerning" figures showed patients were being put at risk.
In some hospitals, over half of patients face delays.
Dr Daniels said the one-hour window was "essential to increase the chances of surviving".
"There is no reason really why it should take longer," he added.
The Sepsis Trust believes there are about 250,000 cases every year in the UK - and more than 50,000 deaths.
'My husband died after six-day delay giving antibiotics'
Last July, Simon Smith went to his local A&E department, at Russells Hall Hospital, in Dudley, West Midlands, with pain in his leg.
He quickly started deteriorating, developing a high temperature and heart rate.
But it was a full six days before he was given antibiotics. By that time, sepsis had taken hold.
Fit and healthy at 51, he fought the infection for four months before he died.
At one point, he was even discharged back home - but his liver was damaged and he was readmitted within a few weeks.
His widow, Hayley, said: "He was so strong - if it had not been for that I think he would have died a lot earlier.
"I am just so angry about the delay giving him antibiotics - that could have made all the difference.
"He had all the signs.
"I've seen notes acknowledging he should definitely have had them on day two - but it didn't happen."
Dudley Group NHS Trust, which runs the unit where Mr Smith was treated, said it had offered its "heartfelt condolences" to his family.
It said it recognised there were "areas of learning" from the case but could not comment further until the inquest into his death had been held.
What is sepsis?
Sepsis is triggered by infections but it develops because of an over-reaction by the immune system.
The infection could come from anywhere - even a contaminated cut or insect bite.
Normally, the immune system kicks in to fight an infection and stop it spreading.
But if the infection manages to spread quickly round the body, then the immune system will launch a massive immune response to fight it.
And this can have catastrophic effects on the body, leading to septic shock, organ failure and even death.
Dubbed the "hidden killer", it is hard to spot as there is not a simple definitive test or obvious symptom.
To counter this, hospitals have been given detailed guidance on how to monitor and treat patients.
It lists symptoms to look out for and says where sepsis is suspected, antibiotics must be given via a drip.
What has BBC News found?
For the past three years, hospitals in England have had to report how well they are identifying and treating sepsis.
A small part of their budget - less than 1% - has been held back in recent years to give them an incentive to take part and improve.
One of the measures is how quickly antibiotics are given.
The BBC has analysed figures from more than 100 hospital trusts, around three quarters of those in England.
They indicate that around 75% of patients got treatment within an hour between January and March.
Performance on wards and in A&E was similar, but A&E departments have improved since early 2017 when about three fifths of patients were started on antibiotics within an hour.
But there were huge variations between different trusts, with a number failing to quickly treat most of their patients in one hour.
Hospitals across the rest of the UK are also expected to treat patients within an hour.
In Wales, 71% of A&E patients and 83% of hospital patients had antibiotics within an hour, alongside checks and tests.
No local figures were available.
Neither Scotland or Northern Ireland could provide any recent figures.
Should anyone wait longer than an hour?
Quick treatment is essential for increasing the chances of survival.
But there are valid reasons for waiting longer than one hour for treatment.
Doctors may be waiting for tests to come back, further assessment may be needed or there could be concern about allergic reactions to certain antibiotics.
It is also very hard to tell after an hour whether the right antibiotics have been given - so hospitals are also expected to review treatment within 72 hours.
But the expectation is that nearly all patients get antibiotics in an hour - in fact over recent years hospitals have had money withheld if they have not done that in at least 90% of cases.
NHS England's Celia Ingham Clark said there had also been an emphasis on improving the way suspected cases are identified by introducing clear screening protocols.
"It's important not to automatically give antibiotics to everyone, instead we want to identify the sickest patients and get them assessed and then quickly give them antibiotics."
What are the symptoms?
- slurred speech
- extreme shivering or muscle pain
- passing no urine in a day
- severe breathlessness
- high heart rate and high or low body temperature
- skin mottled or discoloured
- looks mottled, bluish or pale
- very lethargic or difficult to wake
- abnormally cold to touch
- breathing very fast
- a rash that does not fade when you press it
- a seizure or convulsion