Most broadband providers in the UK "have been forced to cut the headline speeds they advertise when selling deals" because of new UK rules requiring accurate speed claims, according to a consumer advocacy group.

"Eleven major suppliers have had to cut the advertised speed of some of their deals, with the cheapest deals dropping by 41 percent," the group wrote last week.

The analysis was conducted by Which?, a brand name used by the Consumers' Association, a UK-based charity that does product research and advocacy on behalf of consumers.

"BT, EE, John Lewis Broadband, Plusnet, Sky, Zen Internet, Post Office, SSE, TalkTalk, and Utility Warehouse previously advertised their standard (ADSL) broadband deals as 'up to 17Mbps,'" the group noted in its announcement on Saturday. "The new advertised speed is now more than a third lower at 10Mbps or 11Mbps."

"TalkTalk has completely dropped advertising speed claims from most of its deals," the consumer group also said. "Vodafone has also changed the name of some of its deals: Fibre 38 and Fibre 76 are now Superfast 1 and Superfast 2."

“Up to” speeds must be accurate half the time

The new rules were implemented in May by the Committees of Advertising Practice, the UK ad industry's self-regulatory body. Which? said it had been "campaigning for an advertising change since 2013."

Previously, ISPs were allowed to advertise broadband speeds of "up to" a certain amount, even if only one in 10 customers could ever get those speeds, Which? wrote. "But the new advertising rules mean that at least half of customers must now be able to get an advertised average speed, even during peak times (8-10pm)," the group said.

The entry-level speed tiers were apparently the least accurate before the rule change. While advertised speeds dropped the most on entry-level tiers, there were drops in higher-speed tiers as well.

"[A]cross all the deals on offer from the 12 biggest providers, the advertised speeds from 'up to 17Mbps' to 'up to 100 Mbps' had decreased by an average 15 percent," the group wrote. Virgin Media was the only ISP whose advertised speeds went up after the rule change.

In May, just before the new rules took effect, Which? found that "British households are paying for broadband services that are on average 51 percent slower than advertised." The data was based on 235,000 uses of the group's broadband speed test. "[O]n average, customers are paying for speeds of up to 38 megabits per second, but actually only receiving half that (19Mbps)," the group wrote.

No similar rules in US

The US doesn't have similar rules for broadband speed ads, although the US ad industry's self-regulatory body sometimes tells ISPs to cancel misleading ads. In one case, the National Advertising Division urged Comcast to discontinue its claims that it "delivers the fastest Internet in America" and the "fastest in-home Wi-Fi."

The Federal Communications Commission does test in-home speeds and issues reports comparing actual speeds to advertised ones. These Measuring Broadband America reports were issued annually between 2011 and 2016, during the Obama administration, but the Trump-era FCC hasn't issued a new one.