The UK's advertising watchdog is looking into the way so-called "fibre broadband" services are marketed.
In some cases, ISPs are advertising services as fibre that rely on slower copper wires for the final link to a customer's home.
The probe has been prompted by consumer complaints and calls from MP Matt Warman to investigate.
Mr Warman said ISPs were misleading customers by giving them copper when they expected fibre.
In a statement, the Advertising Standards Authority (ASA) said it was acting because of "evolving concerns" about the way fibre broadband services were advertised, and recent changes to government policy, which meant far more people would potentially have access to such services.
Policy papers spelling out the government's view made clear that fibre should be used only to describe services that rely on the faster cables from end to end, it said.
The ASA said it would consider whether ISPs should be able to continue using "fibre broadband" to describe both full and part-fibre services.
"We will be considering whether the use of that term is likely to cause people to be materially misled," it said.
The ASA did not say when it would complete its work but said it would provide an update by the summer.
James Blessing, chair of the council at the UK's Internet Service Providers Association said it had been "working with the ASA on their wider review into broadband advertising to ensure that adverts are understood by consumers and help them make informed choices about the services, products and technologies that best suit their needs."
He said ISPA "looked forward" to helping the ASA's review of what the word "fibre" means in adverts.
Andrew Ferguson, co-founder of the ThinkBroadband website, said "clarity" on advertising would be welcome. "Then we can avoid adverts that promote fibre optic broadband but clearly show a coax cable with a metal core," he said.
Telecoms watchdog Ofcom offers an app that can help consumers check broadband speeds and availability. Commercial sites. such as ThinkBroadband and USwitch, have also made tools that let people check deals and speeds available.