England’s blood donor service has warned blood stocks are still in short supply as it launches a fresh appeal for donors.

Supplies of blood have improved since an “amber alert” in July but there is still a risk that the care of hospital patients could be impacted, officials said.

The number of people booking appointments to donate is dropping off over the coming months, said the service.

Young donors are urged to come forward, especially those with O negative blood type, and more donors of black heritage are also being sought to help treat patients with sickle cell disease.

In July, for only the second time in its history, the NHS warned hospitals that stocks of O-type blood – which can be given to the vast majority of patients – were running low enough to have an impact on patient treatment. This is known as an amber alert.

The NHS said it was due to a “perfect storm” of unfilled appointments at donor centres and increased demand following a cyber-attack, which affected services in London.

At that time, stocks of O negative stood at just 1.6 days, and 4.3 days for all types of blood.

O negative blood is universal and can be given to anyone. It is used in emergencies or when a patient’s blood type is unknown.

O positive blood is the most common type and about 35% of donors have it. It can be given to anybody with a positive blood type and all men and women past childbearing age.

Stocks of O negative blood are now at 7.8 days and overall stocks of all blood types have risen to 8.6 days, said NHS Blood and Transplant (NHSBT), the body that oversees England’s blood donation system as well as the UK-wide organ donor network.

The improvement follows a slight reduction in hospital demand for all blood types.

NHSBT’s Wendy Clark said they were very grateful to donors who answered the call to fill up sessions – and to hospitals for helping out as well – but warned more needs to be done.

She said: “The amber alert can only be removed once we are confident that stocks have reached a strong and sustainable level.

“To achieve this we need donors to keep up the momentum. If you haven’t stepped forward yet, we need you to do so now or in the weeks ahead.”

Many people got in touch with BBC News to say they were struggling to book a slot at their local community or mobile donor centre after last month’s alert.

NHS leaders acknowledge these are often the busiest locations and they are regularly all booked up but they say there are 25 town and city donor centres that often have good availability.

An extra 1,000 weekly appointments have been added to help boost blood supplies.

Jenny Shaw, who is donating blood for the 50th time at Manchester’s Plymouth Grove blood donation centre, said it was good for her, as well as helping other people.

“It gives you a feeling that you’re doing something good for society, for everybody, and it doesn’t matter who they are, you never know who you’re saving and who you’re giving your blood to, but it’s a very nice feeling,” she said.

Blood is donated, tested and processed at the Manchester site, which is unique.

It only takes half an hour of your day, said Alistair Dawson, who is donating his blood for the 85th time.

“You come here, you give a pint of blood, have a drink and go home. It’s almost a routine!”

The collected blood is tested for diseases such as hepatitis, HIV and syphilis, as well as malaria if the donor has been somewhere they may have been exposed to the virus.

Next stop is manufacturing, where the blood is spun at high speeds to separate out red blood cells from plasma.

The blood is then distributed to hospitals across the north of England and sometimes further afield.

The NHS says it takes three donations every minute to provide enough blood to deal with emergencies, childbirth and routine appointments.

With a shelf life of 35 days, blood stocks need to be continually replenished.

There are 80,000 appointments available between now and the end of October, according to NHSBT.