Consumers are being guaranteed minimum speeds of 100Mbps in two new ultrafast broadband services from BT.
Those who signed up to either package would get £20 compensation if their internet download speed fell below 100Mbps, up to four times a year, according to BT.
But only 250,000 homes will be able to access the services – a fraction of all internet-connected premises.
A price comparison site also questioned the "hefty" cost of the services.
The Ultrafast Fibre 1 and 2 packages are now available in 46 locations, from Armley in West Yorkshire to Whitchurch in South Glamorgan and Donaldson in Edinburgh.
They offer speeds of up to 152Mbps and 314Mbps respectively, and both come with speed guarantees.
According to Ofcom, the average internet download speed in the UK stands at 44Mbps.
Similar services are offered by Virgin Media, while relative newcomer Hyperoptic is installing fibre-optic cables that will offer speeds of 1,000Mbs to 80 luxury apartments in London.
'Hefty price tag'
Rob Hilborn, of switching site Broadband Genie, welcomed BT's speed guarantee but said the new services, which will cost from £54.99 and £59.99, were expensive.
"We recommend those that are able to upgrade to these packages first consider a standard fibre package rather than simply jumping up to the fastest package available," he said.
"For most, this will be more than adequate and save you a significant amount of cash."
Andrew Ferguson, co-founder of broadband news site ThinkBroadband, said: "The new service is aimed at those who can already get fairly good speeds, but with its largest competitor, Virgin Media, pushing hard with 200-350Mbps products, BT Consumer is now able to offer something with similar speeds which is probably critical to maintaining market share.
"With the rise of online gaming leagues, having a stable connection for gaming at home is becoming more important. And in some areas Virgin Media has a legacy of being oversubscribed. Therefore, there may be lots of people willing to try an alternate service to see if the latency during gaming is more stable."
The underlying technology behind the services is something known as G.fast. Openreach plans to roll out G.fast to 10 million premises by 2020.
It has a shorter reach than the dominant fast broadband technology – known as VDSL2 – but the benefit is much higher speeds.
According to Ofcom, the UK lags behind other countries in terms of its broadband speeds and reliability.
The regulator says just 840,000 premises have access to full fibre services and 1.1 million still cannot get "decent" broadband of at least 10Mbps.
BT's Openreach division, which maintains the UK's broadband infrastructure, has been criticised in the past for delivering a poor service and favouring BT over rival suppliers.
In 2017 Ofcom ordered BT to legally separate the company so that it could "serve all of its customers equally".
The UK government in December promised to make access to speeds of at least 10Mbps a legal requirement by 2020.
Openreach had offered to carry out improvements according to its own timetable but the government rejected the offer.
At the time, Openreach said it accepted the government's decision and wanted to "get on with the job of making decent broadband available to everyone in the UK".