The US broadcaster is expected to announce the deal to the New York stock market on Thursday 1st or Friday 2nd May 2014 at the latest.
The deal is short of the typically aggressive £700m-plus target originally set by Desmond, the owner of the Daily Express and Daily Star.
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.
It is currently unknown whether Viacom will operate Channel 5's from Viacom International Media Networks (VIMN) UK & Ireland's Hawley Crescent headquarters in Camden Lock, the same building and television studios (then called "Breakfast Television Centre") that former UK breakfast television station "TV-am" used to broadcast from.
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 the US pay-TV company to make a major move into free-to-air television in the UK.
In the UK Viacom International Media Networks has moved into commissioning more original, homegrown shows for its key channels Nickelodeon, MTV, and Comedy Central.
Viacom has invested heavily in entertainment, reality and children's television, which would make it a good fit for Channel 5.
It makes shows such as MTV's structured reality series "Geordie Shore" – which made a star of Charlotte Crosby, who went on to win Channel 5's hit reality series "Celebrity Big Brother".
In addition, along with Channel 5, Viacom invested in "Peppa Pig" via Nickelodeon UK's preschool channel, Nick Jr., which airs in Channel 5's children's programming strand Milkshake and on Nick Jr. UK and Ireland.
Channel 5 shows include the US drama imports "CSI" and "The Mentalist", "Eddie Stobart: Trucks & Trailers", "The Hotel Inspector" and "Extreme Fishing with Robson Green".
Channel 5's children's segment Milkshake features shows including "Peppa Pig", "Fireman Sam" and "Thomas & Friends".
Channel 5 has seven channels – the flagship Channel 5, 5* and 5 USA, a +1, one-hour time-shifted variant for each, and Channel 5+24 – 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.
Viacom declined to comment. Northern & Shell, Channel 5's parent company, said no deal had yet been concluded:
Richard Desmond to announce sale of Channel 5 to US broadcaster Viacom
Owner of MTV, Nickelodeon and Comedy Central channels is expected to pay as much as £450m for Channel 5
Richard Desmond is poised to announce the sale of Channel 5 to the US broadcaster that owns MTV for as much as £450m.
The US broadcaster, which owns channels including Nickelodeon and Comedy Central, is expected to announce the deal to the New York stock market on Thursday or Friday at the latest.
The deal is short of the typically aggressive £700m-plus target originally set by Desmond, the owner of the Daily Express and Daily Star.
But it nonetheless makes for a significant profit on the £103.5m he paid to buy the then loss-making Channel 5 from the German broadcaster RTL in 2010.
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 the US pay-TV company to make a major move into free-to-air television in the UK.
In the UK it has moved 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 would make it a good fit for Channel 5.
It makes shows such as MTV's structured 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 segment Milkshake and on Nick Jr.
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.
Viacom declined to comment. Northern & Shell, Channel 5's parent company, said no deal had yet been concluded.
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