The deal, which values ITN at pounds 106m, brings Carlton and Granada's stake in the company to 20 per cent each and United's to 17 per cent.
United, owner of the Anglia and Meridian ITV franchises, as well as Express newspapers, already owned 5 per cent of ITN.
The announcement ends a seven-year saga at ITN, which has been the subject of negotiations, shifting alliances and bitter battles among leading ITV companies who take the ITN news service.
"It is excellent news that the seven-year controversy surrounding ITN's ownership is resolved," Stuart Purvis, ITN's chief executive, said. "It is particularly good news that it has been resolved in this way."
Carlton and Granada, which held 36 per cent each of ITN until earlier this year, having inherited stakes following their acquisitions, respectively, of Central and London Weekend Television, sold 10 per cent each to Daily Mail & General Trust in April, taking their stakes to 26 per cent each. Yesterday's sale to United brings them down to the 20 per cent limited stipulated in the Broadcasting Act of 1990.
The remaining shareholders of ITN are Reuters with 18 per cent and Scottish Television with 5 per cent.
The Independent Television Commission, which regulates the TV industry, had criticised Granada and Carlton for holding on to their excess shares through "dead-locked" companies, complaining that the arrangements were not in the spirit of the law.
But a Granada source defended the length of time it took to sell off the excess shares, saying it was commercially unfair to be forced to reduce the stake.
The scaling down of the Granada and Carlton positions coincided with negotiations over the renewal of ITN's lucrative contract with the ITV network, under which the news provider was paid pounds 57m a year.