The ticket was for the draw on Wednesday 14 September. The numbers were 16, 23, 35, 42, 44, 58 and the bonus ball was 28.
The winner has until 13 March 2017 to claim the money before it goes towards lottery-funded projects.
A spokesman for organisers Camelot urged players to check "pockets, bags and down the back of the sofa".
Updates on this story and more from Essex
Under Lotto rules, as soon as the jackpot reaches £22m, the prize must be won in the next draw.
Nobody matched all six numbers on 14 September and the £24.5m prize was shared between three winners who matched five numbers plus the bonus ball, two of whom have come forward.
Andy Carter from The National Lottery said: "We're eager to find the mystery ticket-holder and unite them with their winnings.
"We're urging everyone who bought a Lotto ticket in Colchester, whether they are from the town or were just in the area at the time, to check their old tickets again or look anywhere a missing ticket could be hiding."
It is not known if the winning ticket was bought by an individual or a syndicate.
An unclaimed National Lottery winning ticket worth £8.1m was bought in Colchester, it has been revealed.