Richard Desmond is poised to announce the sale of Channel 5 to the US broadcaster that owns MTV for as much as £450m.

US media group Viacom, which owns channels including Nickelodeon and Comedy Central, is expected to announce the deal on Thursday when it reports its latest financial results.

The price tag falls short of the aggressive £700m-plus target originally set by Desmond, the owner of the Daily Express and Daily Star newspapers.

Nonetheless, it represents a significant profit on the £103.5m the media tycoon paid to buy the then loss-making Channel 5 from the German broadcaster RTL in 2010. Desmond turned the business round by slashing costs and bringing Big Brother over from Channel 4, while boosting his other investments by broadcasting regular adverts for his press titles. It is not clear what Viacom will do with Channel 5's £350m-a-year TV ad sales business. But it is thought likely that it will involve Rupert Murdoch's Sky.

BSkyB handles the TV ad sales in the UK for Viacom – the US broadcaster is Sky Media's biggest customer – and it is most likely to incorporate Channel 5's sales into its existing deal.

Desmond based his valuation on 10 times anticipated profits of £70m this year, but a number of analysts had set a top end valuation of £400m to £500m.

The deal would allow Viacom, a big player in the cable network business in the US, to make a major move into free-to-air television in the UK. On these shores its most ambitious broadcasting move in recent years has been

a move into commissioning more original, homegrown shows for its key channels MTV, Comedy Central and Nickelodeon.

Viacom has invested heavily in entertainment, reality and children's television, which fits into Channel 5's current schedules. It makes shows such as MTV's reality series Geordie Shore – which made a star of Charlotte Crosby, who went on to win Channel 5's Celebrity Big Brother.

In addition, along with Channel 5, Viacom invested in Peppa Pig via Nick Jr, which airs on Channel 5's children's slot and on the Nick Jr channel.

Channel 5 shows include the US drama imports CSI and The Mentalist, Edie Stobart: Trucks & Trailers, The Hotel Inspector and Extreme Fishing with Robson Green. The broadcaster's children's strand Milkshake features shows including Peppa Pig, Fireman Sam and Thomas & Friends.

Channel 5 has six channels – the flagship Channel 5, 5* and 5 USA, and a +1, one-hour time-shifted variant for each – some of which could be converted to pay services.

Rivals including BSkyB and Discovery, which was considered the frontrunner to buy Channel 5, have expanded their operations into the free-to-air market. BSkyB is expected to have an involvement in Channel 5 if Viacom pulls off a takeover, by taking over the broadcaster's advertising sales and giving it greater competitive clout against bigger rivals ITV and Channel 4.

Viacom declined to comment. Northern & Shell, Channel 5's parent company, said no deal had been concluded.

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