Brittany Cartwright is working to clear up a tax issue after California tax authorities claimed she owes nearly half a million dollars in unpaid taxes ... TMZ has learned.

According to docs, seen by TMZ, "The Valley" star appears on the California Franchise Tax Board's semiannual "Top 500 Delinquencies" list, which tracks the state's largest outstanding personal income tax debts over $100K. According to the listing, Brittany allegedly owes $463,632.12.

But Brittany says the debt is tied to an old issue with her previous tax filings.

TMZ obtained a letter from Brittany's CPA, Christopher Fank, who says he has handled Brittany's taxes for the past five years and can confirm she has paid all taxes owed during that time.

According to the letter, Brittany's 2019 taxes mistakenly reported "phantom income and tax" under Brittany's name.

The CPA says he is now working directly with the California Franchise Tax Board to correct the issue and claims the agency has agreed to place a hold on the account while Brittany submits proof she does not owe the disputed amount.