The British Horseracing Authority announced the change in protocol after rises in Covid-19 infection rates across the country over the last couple of weeks, with over 180,000 positive tests confirmed on Wednesday.
From January 5, everyone entering the weighing room complex on racedays will have to show evidence of a negative LFT, with all attendees also encouraged to get vaccinated at the earliest opportunity.
The BHA's chief medical adviser, Dr Jerry Hill, said: "Racing's Covid-19 mitigation strategy continues to focus on protecting our business-critical raceday personnel, whose absence due to illness or isolation may result in us struggling to staff fixtures.
"The introduction of mandatory pre-raceday lateral flow testing for weighing room workers is therefore another important step to help safeguard our people and the racing product.
"Vaccination remains an essential part of the fight against Covid-19, reducing the likelihood of serious illness; and combined with regular testing, we are taking the necessary precautions to minimise our chances of catching and spreading the disease in this core work environment."