TKOF WARNING HORN SOUNDED. TKOF ABORTED. FOUND FLAPS IN UP POSITION ON MLG ACR.

Narrative

AFTER NORMAL PREFLT AND BOARDING OF PAX; FLT DEPARTED GATE ON SCHEDULE; STARTED ENGS AND WAS CLRED TO TAXI TO RWY 25 FOR TKOF. I CALLED FOR THE PRETKOF CHKLIST DURING TAXI OUT AFTER THE CABIN WAS RPTED SECURE BY SENIOR F/A. THE CHKLIST WAS COMPLETED WELL BEFORE ARRIVING AT DEP RWY. FLT WAS CLRED FOR TKOF BY TWR; THE BELOW THE LINE PORTION OF THE PRETKOF CHKLIST WAS ACCOMPLISHED WHILE GOING INTO POS AS PER OUR NORMAL PROCS. THE TKOF WARNING HORN SOUNDED AS THE THROTTLES WERE ADVANCED AND WE DISCONTINUED TKOF IMMEDIATELY (ENGS HAD NOT SPOOLED UP) AND TAXIED CLR OF RWY. THE FLAPS WERE UP. BOTH THE COPLT AND I ARE NORMALLY VERY THOROUGH AND ATTN-TO-DETAIL ORIENTED; ESPECIALLY WHEN IT COMES TO CHKLISTS. NEITHER OF US COULD EXPLAIN OR REMEMBER HOW WE BOTH COULD HAVE MISSED THIS VERY IMPORTANT ITEM ON THE CHKLIST; BUT WE DID. AFTER POSITIONING THE FLAPS TO PROPER POS AND DOING THE CHKLIST AGAIN IN ITS ENTIRETY; I MADE A SHORT BUT HONEST PAX ANNOUNCEMENT BEFORE THE FLT CONTINUED W/O FURTHER INCIDENT. AS BEST AS I CAN REMEMBER THERE WERE NO MAJOR DISTRS DURING OUR TAXI OUT. THE WX WAS CLR; NO WIND; AND WE WERE THE ONLY ACFT MOVING AT THE TIME. THE CABIN HAD BEEN SECURED AND THE COCKPIT DOOR WAS CLOSED. BOTH THE COPLT AND I HAD AT LEAST 8 HRS OF SLEEP. COPLT AWOKE EARLY ENOUGH TO RUN PRIOR TO JOINING ME FOR BREAKFAST. IN THIS CASE; EVEN WITH HUMAN ERROR INVOLVED; THE SYS WORKED.