The guidance has been issued by the National Institute of Clinical Excellence (NICE) to its appraisal committees.
Normally, NICE will not approve treatments costing the NHS more than £30,000 for each extra year of life gained by patients.
The new guidance applies to patients with a life expectancy of less than 24 months, who could have a treatment that will extend their life by three months.
Committees can now approve treatments costing the NHS more than £30,000 per quality-adjusted patient year (QALY) if they feel evidence is strong enough that they will extend life.
The recommendation follows a consultation that ended last year and comes into effect today (5 January).