A three-member doping tribunal appointed by the Pakistan Cricket Board (PCB) handed down the verdicts on the pair on Wednesday after hours of deliberations in the eastern city of Lahore.
Akhtar and Asif were sent home from the Champions Trophy in India on Oct 16 after the PCB said the performance-enhancing drug nandrolone had appeared in their samples.
Both had pleaded their innocence, saying they did not take any banned substances knowingly, but they did not challenge the test results and had declined an offer from the tribunal to have their 'B samples' tested.
"We gave a full chance to both the pacemen to fight their cases and after a thorough inquiry we feel they failed to prove their innocence," tribunal chief Shahid Hamid told reporters.
"The process of doping was investigated carefully. We made sure the tests were conducted properly, samples reached the laboratory in Malaysia safely and there was no error in testing in the laboratory.
"Both the players were satisfied and accepted the tests and gave their point of view but after a thorough inquiry and bound by the WADA (World Anti-Doping Agency) and the International Cricket Council (ICC) we reached the decision," said Hamid, a former governor of Punjab province.
The ban may effectively end 31-year-old Akhtar's mercurial career, which has been marked by controversy over his bowling action controversy, indiscipline, injuries and ball tampering.
Akhtar, who has taken 165 wickets in 42 Tests and 133 in 208 one-day internationals, is famously known as the "Rawalpindi Express" for his pace and for his home town.
He launched the fastest electronically measured ball ever bowled - at a speed of 100.23mph (161.3kph) - against England in February 2003 at the World Cup in South Africa.
The 23-year-old Asif, whose career took off early this year before being held up by injuries, has 30 wickets in six Tests and 19 in 17 limited overs internationals.