Hospitals are putting patients' health at risk by failing to test for blood clots at an early stage, according to the healthcare watchdog.
The National Institute for Health and Clinical Excellence (Nice) said many lives could be saved if medics diagnosed and treated blood clots in the legs and lungs more quickly.
It also said diagnosis and treatment of the condition, know as VTE or venous thromboembolism, is patchy and varies significantly across the country.
Nice is advising hospitals in England and Wales to offer blood tests and an ultrasound scan within 24 hours of a patient reporting possible symptoms.
Blood clots in the legs or lungs affected more than 56,000 people in England last year with hospital patients being particularly susceptible because they are in bed for long periods.
Clots in the legs can cause long-term pain, severe swelling and disability, and can be fatal if they dislodge and travel to the lungs.
Hospitals in England are already obliged to check all patients for the risk of developing blood clots.
The advice has been welcomed by the thrombosis charity Lifeblood, which helped develop the guideline.
Medical director Beverley Hunt said: "This is a major advance in improving the quality of care patients receive. It sets a clear standard of care."
Health Minister Simon Burns said: "Already the NHS has dramatically increased screening for venous thromboembolism (VTE), from around just 50% in September 2010 to a world first of 90% by December 2011, leading to better treatment and outcomes for patients suffering this serious and frightening condition.
"However, despite this improvement, we know more can be done in reducing the variation in VTE treatment.
"We expect the NHS to reduce this variation by following this Nice guidance. VTE may have a number of different causes, which means that clinicians treating VTE patients have an excellent opportunity to identify other health problems at an early stage.
"I hope that encouraging clinicians to link VTE and cancer will lead to improvements in the diagnosis of both diseases."