Regulatory filings show Sports Direct has now upped its interest in Debenhams stock from 13.38% to 16.09%.
But instead of buying Debenhams shares outright, the sports firm instead holds "options in stock", which gives it a right to buy shares at a future date.
It also holds "contracts for differences", a form of financial bet on the future Debenhams share price.
Sports Direct first revealed an interest in Debenhams more than three years ago.
Sports Direct has 420 UK stores and owns several brands including Dunlop, Slazenger, Everlast and Karrimor.
It was taken public by Mr Ashley in February 2007 and is a member of the FTSE 250, having spent two-and-a-half years years in the FTSE 100.
Debenhams has 240 stores in 27 countries and reports its half-year results on 20 April.
Shares in the company were trading as high as 81.55p last April, but now stand at 54.40p.
Mike Ashley's Sports Direct has raised its stake in High Street department store chain Debenhams.