An urgent plea for blood donors to come forward has been issued by the health secretary as stocks required to carry out critical operations run low.
Steve Barclay said the NHS is running on three-and-half days' worth of blood reserves - whereas it normally aims to hold six days.
If supplies drop to two days, the NHS will trigger an "amber alert" which could lead to hospitals having to cancel planned operations - at a time when the health service is already grappling with record waiting lists for treatment.
Mr Barclay, who donated blood at a hospital in Stratford in east London today, insisted there is still "plenty of supplies" with levels currently 50% above amber.
But he said it was important to be "proactive" in calling for donations to stop reserves plunging further over the summer holidays.
He said: "Stocks of blood supplies are lower than we would like. We still have plenty of supply, but summer is a traditionally a time when donations are lower than they would be at other points of the year."
Mr Barclay said supplies were running low mainly due to COVID and staff absences.
The NHS has never issued an amber alert over blood supplies before.
But earlier this month, the Health Service Journal reported that it was close to doing so as stock levels had become "severe".
Mr Barclay said he did not want to wait for supplies to reach critically low levels as he urged donors to come forward now.
He said there was a particular need for O, B- and A+ types, to help re-build low blood stocks and keep services running, as well as donors from ethnic minority backgrounds.
Around 800,000 people currently donate blood each year.
But the NHS says one million more blood donors are needed over the next five years in order to save lives, with a particular need for black African, black Caribbean and younger donors.