Virgin Media is to challenge the Competition Commission's ruling that BSkyB must reduce its stake in ITV, saying it does not go far enough.
The regulator was concerned BSkyB could influence decisions at ITV
The Commission ruled last month that BSkyB must cut its ITV shareholding from 17.9% to below 7.5%, a decision upheld by the government.
Virgin said BSkyB should be forced to reduce its stake still further.
Last week, BSkyB launched its own appeal against the decision, arguing its ITV stake is not anti-competitive.
The commission ruled in January that BSkyB's stake in ITV thwarted competition and allowed it unfair influence over its television rival.
Virgin said the Commission's ruling had made "significant errors".
BSkyB, controlled by Rupert Murdoch's News Corporation bought its 17.9% stake in ITV in November 2006 for £940m.
The move was widely seen as a means to prevent Virgin's plans to merge with ITV, something denied by BSkyB.
If BSkyB is forced to reduce its ITV stake it could lose about £250m, as ITV's shares have fallen over the past year.
BSkyB argues that competition in the UK television market remains "as vigorous as ever".
"A merger has not taken place, Sky and ITV are distinct entities with independent strategies and Sky could not block a shareholder resolution without voting rights."
Both Virgin and BSkyB have lodged their appeals with the Competition Appeal Tribunal.