AG Barr and Britvic said the proposed tie-up will create one of Europe's leading soft drinks firms with annual sales of more than £1.5 billion.
However, the merger will come at the expense of up to 500 jobs after the two companies forecast a reduction of between 8% and 12% in their combined headcount of just over 4,000 people. It was not immediately clear where the job losses would come.
Hertfordshire-based Britvic, whose brands include Robinsons, Fruit Shoot, R Whites and Tango, has around 3,300 staff. AG Barr, which dates back to 1875 and also makes Tizer and Rubicon, has just under 1,000 employees.
It was confirmed today that Britvic shareholders will own 63% of the new company - to be called Barr Britvic Soft Drinks - with AG Barr holding the rest. The deal is still subject to shareholder approval.
The new company will continue to base its head and registered offices in Cumbernauld.
Cumbernauld and Kilsyth MSP Jamie Hepburn has welcomed the news but says clarity is now needed on how many jobs will be lost in Scotland.
He said: “It is a positive development that the new company want to retain their base in Cumbernauld, obviously seeing the business benefits of remaining head-quartered north of the border.
“What everyone will now be looking for is clarity on where the job losses will be coming from. With the group's operational headquarters planned at Britvic's existing head office in England this creates some doubt.
“The employees of AG Barr in Cumbernauld, many of whom have worked for the company for a number of years, already have the relevant skills to enable them to excel at that location and as such should be viewed by Barr Britvic as an invaluable asset which they should do their utmost to retain in its full capacity.”
Barr, which is based at Cumbernauld, North Lanarkshire, has produced Irn-Bru from a secret recipe for more than 130 years.
Chairmanship of the company passed outside the family for the first time in 2009 when Robin Barr ended his 31-year tenure.
He remains on the company's board as a non-executive director and is one of just three people to know the formula of 32 ingredients used in the drink.
Barr Soft Drinks was started in 1875 when former chairman Robin Barr's great grandfather Robert Barr established a soft drinks business at the company's first factory at Burnfoot Lane in Falkirk.
A second Barr soft drinks business was established in Glasgow in 1887 by Robert Barr's son, Robert Fulton Barr. His business was taken over by his brother Andrew Greig Barr - from whom the company gets its name - five years later in 1892. In 1901 both Barr soft drinks businesses jointly launched their original recipe Iron Brew - as it was spelled then.
The recipe for Irn-Bru is known by only three people in the world - former chairman Robin Barr, his daughter, company secretary and legal affairs manager Julie Barr, and one other AG Barr board director whose identity remains confidential.
The phonetic spelling Irn-Bru was introduced in 1947 following concern over proposed changes to food labelling regulations.
AG Barr employs around 980 staff working across 11 sites in the UK. Its head office is in Cumbernauld, it has two regional sales, marketing and administration offices in Middlebrook near Bolton and Wembley in London, and it has sales branches in Newcastle, Moston, Sheffield, Wednesbury and Walthamstow. Four production sites are at Cumbernauld, Forfar, Tredegar and Pitcox.
As well as Irn-Bru, the company owns the Tizer, Barr's Originals, Orangina and Rubicon brands.
Irn-Bru is dubbed Scotland's "other national drink" where AG Barr sells 12 330ml can equivalents of Irn-Bru every second.
The company can trace its origins back to a chemist in Chelmsford in the mid-19th century but it was not until 1949 that a range of juices were launched under the name British Vitamin Products.
In 1971, The British Vitamin Product Company formally changed its name to Britvic.
Britvic acquired the UK licence for Dr Pepper in 1982 and the Tango brand in 1986.
The company was awarded a 20-year bottling arrangement for Pepsi and 7UP in Great Britain in 1987.
Robinsons brand was bought by Britvic in 1995 from Reckitt & Colman.
Britvic floated on the London Stock Exchange in 2005.
The company's head office is in Hertfordshire and it collectively employs around 3,300 staff, and sells 1.9 billion litres of soft drinks each year.