Holidaymakers who book hotels and flights from different firms but via a single travel intermediary will now get the same protection as those buying traditional package holidays.
Package holidaymakers are both financially and legally covered if things go wrong.
But those who book different elements of their holiday separately have been less well protected.
The new EU regulations attempt to close the gap in consumer protection.
"Proper package holidays are the gold standard of travel," Simon Calder, travel correspondent at The Independent, told the BBC.
"By booking flights and accommodation in a single transaction from a tour operator, you transfer all the risk to the company. They have to sort out things like cancelled flight or an overbooked hotel.
"They also have to provide a refund or compensation as appropriate. But with the growth of online travel agents, people have been buying what they think is a package holiday, only to discover when it all goes wrong that the travel firm denies all responsibility.
"With these new changes the basic rule is - if it looks like a package holiday, with flights, accommodation and possibly a rental car all included in a single transaction, then it should have with all the prodigious consumer protection that comes with a proper package."
He said that already some companies such as On The Beach and Travel Republic had changed their terms to reflect the change.
However he warned that if you are asked to pay separately online for flights and accommodation, it is likely you won't get any extra protection.
The government said the new rules would protect an extra 10 million package travel holidays a year.
The Package Travel Regulations 2018 were prompted by the growing number of people who book flights, hotels and other tourist services, like car hire, online. Last year 83% of British people booked a holiday over the internet.
The new rules will also apply to people who go into a High Street travel agent and pick and choose separate elements of their holiday and pay for them all together.
Until now mix-and-match holidays were financially protected, if an airline went out of business, for example.
However, if a holidaymaker had a grievance with a hotelier, for example, they would have had to pursue any legal action themselves.
Widening the definition of a package holiday means that from now on in such a situation the travel company is "responsible for making sure that you get the holiday you paid for", the Association of British Travel Agents (Abta) said in its guidance on the new rules.
"If something isn't provided or isn't as expected, and your travel company or its suppliers is at fault, they will need to sort this out for you," it added.
Travel litigation specialist Joanne Brine of JMW Solicitors said: "Should anything happen while you're on holiday - such as an accident or injury on hotel property - subsequent claims will be more straightforward to deal with, since the operator you booked with will hold liability if travel services aren't provided with reasonable skill and care.
"Plus, if an operator goes bust, you're guaranteed to receive a full refund or, if you're already abroad, to be brought home. Added legal protection also gives the right to help if weather conditions or industrial action hamper your plans.
"Package holiday regulations have finally been dragged out of the dark ages," said Which? travel editor Rory Boland.
"These new rules mean far more holidays will be classed as packages, giving holidaymakers protection when something goes wrong."
'Linked travel arrangements'
However, there are instances when a holidaymaker might think they are fully covered when they are not.
That is because there are separate rules for "linked travel arrangements" online.
This is when someone buys a flight on an airline's website, for example, and pays for it.
They might be encouraged to go to a separate accommodation site, either immediately or within the next 24 hours, and they pay for a hotel or car rental separately.
This arrangement is not classed as a package. It does not have any legal protection and the financial protection is at a lower level than for packages.