Health officials are encouraging people to "look before they flush" in a bid to reduce the number of kidney and bladder cancers.
Having blood present in urine is a symptom in over half of bladder cancers and almost a fifth of kidney cancers, Public Health England (PHE) said.
The health body's Be Clear on Cancer campaign launches on Tuesday and is encouraging people to visit their GP if they notice "blood in pee" - even if it is just once.
Doctor Jenny Harries, Public Health England's regional director for south of England, said: "It's vital that we all do our bit to raise awareness that blood in pee could be an early sign of bladder or kidney cancer, and encourage anyone with this symptom to go to their GP.
"We know that people don't always immediately visit their doctor if they spot blood in pee, which can be for a number of reasons - some might ignore the symptoms, especially if it only happens once, or may pass the symptom off as cystitis. If you do notice blood in your pee, don't wait for it to happen again before getting it checked out, visit your GP straight away."
Dad's Army star Ian Lavender, who suffered from bladder cancer, is supporting this year's campaign. "It's a simple message 'look before you flush' and make sure you go and see your GP if you notice blood in your pee," he said.
"Spread the word, someone you know might have this symptom and reminding them to get it checked could save their life - it saved mine, and I'm 70 and still happy to be working."
Around 17,450 people in England are diagnosed with bladder or kidney cancer every year and around 7,600 people die annually, PHE said.
PHE's new campaign is aimed at men and women over the age of 50 as more than nine in 10 of these cancers are in people in this age group.
:: For more information visit nhs.uk/bloodinpee