Satellite television company BSkyB has bought a 17.9% stake in ITV for £940m ($1.8bn) - but said it had no plans to make a full bid for the UK company.
ITV is one of Europe's top broadcasters says James Murdoch
BSkyB bought 696 million shares at 135 pence each, saying ITV had substantial potential for long-term value creation.
ITV has been under pressure after being hit by declining advertising and is currently without a chief executive.
Cable TV operator NTL approached ITV about a merger last week, while reports suggest Germany's RTL may make a bid.
The Financial Times said the German owned broadcaster was considering a possible £5bn bid for ITV.
BBC Business editor Robert Peston said the broadcasting regulator would not allow BSkyB to buy more than a 20% stake in ITV and that the move was seemingly designed to block an NTL takeover.
"This is an astonishing spoiling tactic," he said.
BSkyB chief executive James Murdoch said it believed ITV was one of Europe's premier broadcasting businesses.
"We want our investment to give ITV the room it needs to put the best new management in place and explore the full range of options to create value for all shareholders," Mr Murdoch added.
ITV acknowledged BSkyB's comments and said it "looked forward to discussions".
The 135p-a-share offer is 16.6% more than the ITV closing price of 115.75p, which had itself climbed 0.9% on Friday on the RTL speculation.
RTL, Europe's largest broadcaster, is owned by German media firm Bertelsmann.
Its broadcasting interests include 34 television and 33 radio stations in 11 countries including TV station Five in the UK.
NTL - which has recently merged Virgin Mobile and Telewest - said last week it had "scheduled an initial conversation" with ITV but that the process was at a very preliminary stage.
Head of equities at Hargreaves Lansdown stockbrokers, Richard Hunter, said BSkyB's move was "shrewd".
"A stake of 17.9% is not quite enough to scupper a deal, but it will make life a lot more difficult for NTL or RTL," he said.
"For a start it would give BSkyB an insight into NTL or RTL's strategy as BSkyB would have to be consulted as the company's biggest shareholder.
"And ultimately they will have a big say in who gets to control ITV."
In August it said that revenue at its main ITV1 channel had fallen 8% in the first six months of 2006, to £654m.
ITV is looking for a new chief executive following the summer exit of Charles Allen.