All iPlayer users must now own a TV licence, regardless of whether or not they watch programmes live.
The new rules, which will close the so-called “iPlayer loophole”, came into effect on Thursday morning.
Previously, only viewers who were watching shows as they were being broadcast required a licence.
That meant it was legal to watch content after broadcast via the catch-up service without paying the annual £145.50 fee.
The changes to the rules are expected to particularly affect younger people, who are more likely to consume content on smartphones or tablets than via traditional television sets.
TV Licensing has already begun making students more aware of the changes, many of whom will be affected at the start of the new academic year.
It says that “in limited circumstances”, students can be covered by the licence at their parents’ address and advises them to check its website for more details.
It is unclear how the new rules will be enforced. Viewers are now prompted to declare if they have a TV licence when they try to watch any content on iPlayer.
A TV Licensing spokesperson said: “We know the vast majority of people are law abiding and would anticipate those who need a licence for the first time will buy one.
“We have a range of enforcement techniques which we will use and these have already allowed us to prosecute people who watch on a range of devices, not just TVs.”
A BBC spokesperson said: “At present, a pop-up window appears asking viewers to confirm they’ve got a TV licence when they click to play live BBC content on iPlayer, and from today, that will update to include on-demand BBC programmes on iPlayer.
“The vast majority of households – around 94% – are already licensed so this change will not affect them. You still need a licence to watch or record live TV on any channel.”
Damian Collins, acting chair of the Culture, Media and Sport Select Committee, told the BBC he was not certain how the new rules would be enforced.
He told Radio 4’s Today programme: “The BBC haven’t set that out and this is undoubtedly something we’ll ask the director general about the next time he’s in front of the committee.”
Collins added he would be open to the corporation exploring a variety of ways to enforce the new rules.
“A good way around this could be having a pin number to access iPlayer and online services,” he said.
“There’s been talk about having a system so you could access iPlayer from abroad. I think this could open up not just a better service for BBC viewers and customers but also additional revenue for the BBC in the future as well.”
The change comes after the government said it wanted to modernise the current system, so those watching catch-up TV do not get “a free ride”.
The new rules apply to all devices used to access iPlayer – including laptops, smartphones, tablets, TV streaming devices and games consoles, as well as through third-party services such as Sky, Virgin or BT.
Those who already have a TV licence will not be affected.