The WHO report, the most comprehensive analysis of the issue to date covering 114 countries, said resistance was now emerging in every part of the world, including in 'last-resort' drugs.
Experts said the serious threat posed by a lack of effective antibiotics was 'happening right now', and was no longer a prediction for the future.
To reduce the spread of resistance, they called on healthcare professionals to prescribe antibiotics only 'when they are truly needed' and ensure the correct antibiotics are used.
Dr Keiji Fukuda, WHO’s assistant director-general for health security, said: 'Without urgent, coordinated action by many stakeholders, the world is headed for a post-antibiotic era, in which common infections and minor injuries which have been treatable for decades can once again kill.
'Effective antibiotics have been one of the pillars allowing us to live longer, live healthier, and benefit from modern medicine. Unless we take significant actions to improve efforts to prevent infections and also change how we produce, prescribe and use antibiotics, the world will lose more and more of these global public health goods and the implications will be devastating.'
Resistance 'very widespread'
The report, Antimicrobial resistance: global report on surveillance, focused on seven bacteria responsible for common diseases such as sepsis and diarrhoea.
Carbapenem antibiotics - a last-resort treatment against the intestinal bacteria Klebsiella pneumoniae, a major cause of hospital-acquired infections such as pneumonia - are now ineffective in over half of patients treated with the drugs.
Resistance to fluoroquinolones for UTIs is 'very widespread', while 10 countries have confirmed treatment failure with third-generation cephalosporins, the last-resort treatment for gonorrhoea.
The WHO called on patients to help tackle resistance by using antibiotics only when prescribed by a doctor, completing courses and not sharing or storing leftover prescriptions.
It said more action was needed to prevent infections to reduce the need for antibiotics, and for the development of new diagnostics and antibiotics.