The Lake District and Yorkshire Dales national parks have been extended by 188 square miles, with campaigners hailing a "historic day".
The park boundaries have grown within touching distance either side of the M6 motorway, creating a band of protected land across the north west of England.
Originally designated in the 1950s, the expansion of the borders aims to boost rural tourism.
The Yorkshire Dales has grown in size by 24%, with the Lakes increasing 3%.
The move, announced by the Department for Environment, Food and Rural Affairs in October, sees the Dales stretch to the north and west and the Lakes to the east and south.
Most of the Yorkshire Dales park is in North Yorkshire, but 28% of its land is now in Cumbria and 1% in Lancashire.
Despite the change in geography the Yorkshire Dales National Park Authority said the government had no plans to change the park's name, which was written on a designation order in 1954.
Park extension areas
- An area in the east from Birkbeck Fells Common to Whinfell Common
- An area in the south from Helsington Barrows to Sizergh Fell, an area north of Sizergh Castle and part of the Lyth Valley
- The north includes parts of the Orton Fells, the northern Howgill Fells, Wild Boar Fell and Mallerstang
- The west extends to Barbon, Middleton, Casterton and Leck Fells, the River Lune, and part of Firbank Fell and other fells to the west of the River Lune
Natural England first suggested the extensions in 2009, with a public inquiry launched in 2013.
After hearing more than 3,000 objections and representations, it was recommended the extensions be approved.
Mark Corner, from the Yorkshire Dales Society, said: "We're very excited, we have been working on this for many years and campaigned and lobbied wherever we can.
"Some of the boundaries were picked on an arbitrary basis in the 1950s, so to join it up makes an awful lot of sense. It now should be on everyone's bucket list."
The Friends of the Lake District charity said it was "a historic and very rare day".
Policy officer Alison Lax said: "These areas have always been special and valued in Cumbria, but now it adds a level of national recognition.
"It's unfinished business, as they really should have been part of the parks in the first place. The landscape's qualities are on a par with everything that was included in the parks previously."
The combined area of parks makes it the largest area of national park land in England, the charity said.
John Welbank, who runs a planning consultancy for rural businesses, said the move was a boost for tourism but would have a "negative impact on new start-ups".
"When they try and develop businesses and property, most people's attitudes will change drastically as they won't realise the planning implications until they come up against it," he said.
"There are implications for business development projects as it will become significantly harder."
In the Yorkshire Dales, the extension means the number of farms in the national park has gone up 32% to 1090.
When the extension was announced, the Country Land and Business Association (CLA) criticised the move, saying it ignored the views of farmers and landowners.
Greg Stephenson, from Hipping Hall hotel, Cowan Bridge, described the business now being inside a national park "a good thing".
He said: "Leck and Cowan Bridge have been overlooked by visitors because it's a beautiful area, but sits on the fringes between the old borders of the Lake District and the Yorkshire Dales.
"It will now become part of a distinct area which is recognised internationally."
On the topic of a small corner of Lancashire gaining a Yorkshire title, he said: "I'm sure there will be a few people around here who have a tongue-in-cheek moment, but I think most will be happy to be part of this well-established brand."
A Lancastrian's light-hearted take on the Yorkshire Dales entering Lancashire
In the spirit of the age, and after much thought, I have now decided to start a petition to rename the Dales National Park (can't bring myself to use the Y word) as the 'Northern Powerhouse Park of the People'.
The powers that be have tried in vain for over a number of decades to rename parts of the Red Rose county.
I can only imagine the area to be added to the national park must be of a far superior standard to any of the current acres.
I have enlisted the help of a stout Red Rose fellowship and we march at dawn to dig the ditch and erect the fence to beat back the advance of our meddling eastern cousins.
Stephen Lowe, Lancashire Outdoors presenter for BBC Radio Lancashire