The majority of UK broadband providers have been forced to reduce their advertised top speeds following tougher rules.

Analysis conducted by consumer group Which? found that 11 of the biggest broadband providers have had to cut the advertised speed of some of their deals since the new rules came into effect in May.

The advertised speed of some the cheapest deals dropped by an average 41 per cent, according to the research.

Providers including BT, John Lewis Broadband, Plusnet, the Post Office, Sky, TalkTalk, Utility Warehouse, EE and Zen Internet have reduced the speeds they claim to provide.

Previously, providers had been allowed to advertise top speeds as long as they were available to just 10 per cent of customers, resulting in complaints from consumer groups, the government and the public.

→ READ MORE: UK's connectivity 'simply not acceptable' says US tech VP

"Customers will now have a much clearer idea of the speeds that can be achieved when they are shopping around for broadband," said Alex Neill, Which? managing director of home services.

"For those still struggling to get a reasonable speed or connection, the government must press ahead with its crucial plans to deliver the service that broadband customers need, without it costing them the earth."

The new rules from the Advertising Standards Authority (ASA) now require home broadband providers to ensure that at least half of their customers can achieve advertised speeds at peak time.

A National Infrastructure Commission spokesman added: "Our National Infrastructure Assessment, the first of its kind for the UK, endorses a move to full fibre broadband which could help guarantee that customers get the speeds they pay for – as much as 1,000Mbps.

"Today’s findings demonstrate the slower broadband speeds that households and businesses are actually working with. But if we are to make a lasting difference, as well as tackling the immediate issues, we need to invest properly for the future."