Football Pools company Sportech said £95 million is now at stake in its row with the taxman over the Spot the Ball competition.
The Liverpool-based company won a court battle against HM Revenue and Customs in March after arguing it was incorrectly charged VAT sales tax on its competition between 1979 and 1996.
But HMRC is fighting the ruling and an appeal will be heard in April. Sportech said its VAT claim has increased from £80 million to £95 million, including interest.
The dispute centres on whether Spot the Ball - where players win a prize if they correctly identify where a football is in a photo - is a game of skill or chance.
The company scored a victory in March when the High Court's First-tier Tribunal's Tax Chamber ruled the once-popular newspaper promotion was a game of chance, and therefore not eligible for VAT.
Sportech said it "remain(s) confident in our arguments and a successful outcome of the appeal", but is exploring its options to "secure financial benefit" if the appeal goes against it.
The group also revealed a dip in half-year profits and revenues as p unters spending more on the Football Pools failed to offset shrinking numbers playing the 90-year-old game.
Football Pools customers numbers slid 11% year-on-year to 350,000 at the end of June - a low point in the football season - but it is trying to broaden the game's appeal by teaming up with France-Pari for an imminent launch in France.
Average weekly customer spend increased to £2.61 from £2.55 a year earlier, but the game's revenues fell to £20 million from £21.7 million a year earlier and underlying earnings dipped 6% to £8.2 million.
The group's underlying earnings for the first six months of the year were helped by 22% profits growth in its racing arm, which has operations in the US, the Netherlands and Germany.
But pre-tax profits slumped to £1.9 million from £3.2 million a year earlier. Revenues dipped 1% to £57 million.
Sportech said trading was "disappointing" in its loss-making e-gaming arm, after losing the rights to use the Littlewoods brand last year and switching brands and products onto a single offering from its partner Playtech.
However, it has grown e-gaming active players to 24,000 from 20,000 and said it expects to return to profit in the second half.
Sportech lodged its claim against the taxman in March 2009.
The group traces its roots back to 1923 with the birth of the Football Pools.