The Battle of Carham has long been regarded as establishing that the River Tweed was the border between the two countries instead of the Forth.
Experts have struggled to establish whether the battle was fought in 1016 or 1018 and its exact location in Northumberland, although it was fought beside the Tweed.
But following a meeting between Battlefields Trust member Clive Hallam-Baker, Scottish Borders Council's archaeology officer Dr Chris Bowles and Dr Alex Woolf, a senior history lecturer at the University of St Andrews, it has been agreed the anniversary commemoration should take place in 2018.
A steering group involving members of the local community is due to be set up so that a series of events can take place in 2018 marking the battle's 1000th anniversary.
Mr Hallam-Baker, who lives in nearby Cornhill-on-Tweed and organised the 500th Flodden commemoration last year, said:"The consensus of the meeting that it was probably 1018 and it has been decided to go with that year.
"The good thing about it is that it gives us another two years to plan the events."