By Rory Cellan-Jones
Technology correspondent, BBC News
The BBC's iPlayer made its public debut in 2007
Britain's biggest broadband supplier has been accused of limiting download speeds on its cheapest package without giving users a clear warning.
BT Broadband cuts the speed users can watch video services like the BBC iPlayer and YouTube at peak times.
A customer who has signed on to an up to 8 megabit per second (MBPS) package can have speed cut to below 1Mbps.
A BT spokesman said the firm managed bandwidth "in order to optimise the experience for all customers".
The BBC said it was concerned the throttling of download speeds was affecting the viewing experience for some users.
Customers who opt for BT's Option 1 broadband deal will find that the speed at which they can watch streaming video is throttled back to under 1Mbps between 1700 and midnight.
The BBC iPlayer works at three different speeds, 500Kbps, 800Kbps, and 1.5Mbps, depending on the speed of a user's connection. There is also a high definition service which requires 3.2Mbps.
Sources at the BBC said the effect of BT's policy was to force viewers down on to the 500Kbps service, which can make the viewing experience less satisfactory.
Andrew Griffiths, a BT customer in south London, said he had noticed a drastic slowdown in the speed of his BT internet connection in the evenings.
He said: "From about 1730 onwards something like BBC iPlayer becomes impossible. It's fine during the day but come the evening, it's a no go."
Mr Griffiths explained he was on the basic package, and was promised a connection of between 5Mbps and 8Mbps.
"We get nowhere near that. We've tried to contact them numerous times to complain but without success."
In a statement, the BBC said: "While customers listening to audio and lower quality video streams would be unaffected, we are concerned that at peak times some customers' higher quality video streams may be interrupted by buffering before falling back to a lower-quality version.
"This would suggest that traffic identified as BBC iPlayer traffic is being throttled back, thereby limiting the bandwidth used up by the service on slower connections."
BT has said its speed limit is explained on its website, on a page describing its Total Broadband Fair Usage policy.
The clause in question says: "We do limit the speed of all video streaming to 896Kbps on our Option 1 product, during peak times only."
Steve Weller, from price comparison site USwitch.com, said BT was not alone in having fair use policies that were difficult to find.
He said: "Companies operating a traffic management policy usually hide their terms deep in the terms and conditions, making it difficult for customers wanting to get out of a contract."
The BBC launched a HD iPlayer earlier this year
Sources at the BBC said their data showed that speeds for BT's iPlayer users was reduced to around 700kbps at peak times.
The policy is not explained in the terms and conditions users see when they sign up to BT Broadband.
The BT website describes the Option 1 package as best for "high-speed broadband at home". There is a download cap of 10 gigabytes a month, but the advertising promises up to 8Mbps, and BT says "we'll offer the best speed available on your line".
But in a statement BT said it was not capping speeds down to 700kbps on iPlayer.
The firm said: "Where we manage bandwidth, we do so in order to optimise the experience for all customers, whatever they want to do online.
"We believe there is a real issue that content owners like the BBC need to address and we are currently in discussions with the BBC executive to ensure that our customers get the best possible experience in the future."
A number of major broadband suppliers limit speeds at peak times for some users, adopting what are known as traffic shaping policies.
Last year the regulator Ofcom brought in a voluntary code of practice on broadband speeds, which calls on Internet Service Providers to provide clear information about speeds when users sign up.
Ofcom said the code did not ask providers to make fair usage policies clear at point of sale, but they must be made clear on their websites.
Mr Weller, from uSwitch.com, said BT was not the only internet service provider trying to cope with growing demand by throttling back speeds.
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