The move - which values the company at pounds 371.7m - is the latest in a series of mergers in the industry, which has seen Granada Group this week bid for Yorkshire-Tyne Tees Television (YTTV), and Grampian Television recently merge with Scottish Media Group.
Chris Rowlands, chief executive of HTV, said the deal had only been negotiated in the last fortnight and had been triggered by Granada's offer for Yorkshire. Mr Rowlands, who will be leaving the company once the deal is complete, said HTV's shareholders would be pleased by the "exceptional price" offered by United. United was forbidden from bidding less than 420p a share until 1 November, which would have marked the first anniversary that Lord Hollick's media company upped its stake in HTV to almost 30 per cent at the same price.
When Mr Rowlands joined the company in 1992, the share price was at an all-time low of 14p.
Roger Laughton, chief executive of United Broadcasting & Entertainment, said the company was "keen to maintain and develop" HTV's broadcasting and rights business. HTV has over the past few years built up a successful rights business, Harvest Entertainment.
Mr Laughton added that there was a "compelling strategic case" for the acquisition. United also owns the Anglia and Meridian franchises, and the latest purchase will enable Lord Hollick's group to challenge Carlton Communications and Granada Group more effectively.
Last night, just before the close of trading, United News & Media bought 18.8 million shares in the market to underpin its bid. The purchase took United's shareholding to 51 per cent.
Carlton Communications, which had been tipped by analysts as another potential suitor for HTV, is now unlikely to launch a counter-bid.
City analysts added last night that United's move made it highly likely the company would vote its 14 per cent stake in YTTV in favour of the offer for the company made by Granada.
Most industry observers agreed that HTV had secured a "good price", which represented a 40 times multiple on historic earnings. Other deals in the sector have gone for around 28 to 30 times historic earnings.
Lorna Tilbian, media analyst at Panmure Gordon, said United may now be more interested in challenging the Independent Television Commission on its decision this week to award the licences for digital terrestrial television to British Digital Broadcasting. United was part of the rival consortium, Digital Television Network. However, United may decide instead to buy out Yorkshire's 40 per cent stake in S4C Digital Networks (SDN), which is bidding for one of the digital terrestrial multiplexes.
Shares in HTV soared 84.5p to 415p yesterday, while shares in United closed 10p lower at 714.5p
The announcement follows swiftly on the heels of news earlier in the month that Scottish Media Group was in talks about acquiring its neighbour, Grampian Television for pounds 105m. ITV companies had been bracing themselves for a further round of consolidation for a while, after the 1996 Broadcasting Act revised the rules on media ownership.