Adele Fan's Leap Day Proposal FAILS

Adele kicked off her world tour by helping a female fan take a leap of faith ... dropping a Leap Year proposal on her boyfriend in the middle of the concert -- but it did not go as planned.

No one will question the power of Adele's voice, but during Monday night's show in Belfast ... even that wasn't enough to convince the reluctant dude to say "yes" to his gf.

Watch ... things got uncomfortable when the crowd started booing the bf for beating around the bush after Adele and his gf single him out in the crowd.