Webster, 25, had accepted a four-year ban in December 2017 after he tested positive for banned substance ostarine. At the time, Webster said he spent his life's savings trying to prove his innocence.
UKAD said they received information that Webster received payment for coaching three athletes during his ineligibility period when he was not supposed to participate or assist athletes in any capacity.
Webster's ban, that was set to end in June 2021, has now been extended to June 13, 2024.
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"Athletes and athlete support personnel must be fully aware that when they break the rules and receive a sanction, UKAD will continue to monitor their activity, and any prohibited involvement in sport will be punished accordingly," UKAD chief executive Nicole Sapstead said in a statement.
"Hopefully this case serves as a warning to those serving bans that any athletes committing further violations during the term of that ban will result in additional and significant sanctions."
Webster finished 14th in the 94kg category at the 2016 Olympics in Rio de Janeiro. (Reporting by Rohith Nair in Bengaluru Editing by Christian Radnedge)
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