Global, owner of the Capital, Heart and Classic FM stations, is understood to have agreed a deal worth an estimated £70 million with Guardian Media Group for its radio business.
GMG Radio was launched in 1999 and has a total share of 4.5% of the radio listening market, including the five regional stations in the Real Radio network and the national Smooth network.
Global said that GMG Radio will continue to operate separately from its radio business whilst a regulatory review is conducted. However, Global will continue to represent GMG Radio's national airtime sales, as it has done since 2006.
Announcing the purchase yesterday (June 25), Global Group founder and executive president Ashley Tabor said: "We are pleased to have concluded this deal, and once the required approvals have been received, we will look forward to welcoming the GMG radio stations into the Global family.
"We believe that this is a very strong business with brands and assets that are highly complementary to those of Global Radio."
GMG chief executive Andrew Miller added: "Our portfolio of investments that lie outside our core national newspaper business exist to underpin the long-term financial and editorial integrity of The Guardian. If we believe that best value for the Group lies in the disposal of a non-core asset, then we will do so.
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"The talented team at GMG Radio have been very successful in building strong brands, which have been attractive to listeners and advertisers and have attracted considerable external interest.
"We believe that this transaction gives the business an excellent opportunity to develop and flourish as part of Global Radio. I would like to thank everyone at GMG Radio for their outstanding achievements over the last few years and wish them every success for the future."
GMG Radio chief executive Stuart Taylor will leave the company with "immediate effect" after the takeover, and Global's managing director of regions, Mark Lee, will be seconded to lead the business.
However, rival commercial radio groups, including Absolute Radio, Talksport-owner UTV Media and Bauer Media, have expressed concern over the potentially negative impact on the £450m UK radio market following the deal.
In a statement, Talksport chief executive Scott Taunton said: "This merger would leave the enlarged Global Radio controlling more than 50% of commercial radio revenues, and more than 50% of listening in key markets like London and Manchester.
"The competition authorities should see this for what it is, an attempt to achieve an unassailable position of market dominance. Simply put, the proposed merger must not go ahead."
He confirmed that his company was consulting with legal advisers and preparing its submission to the Office of Fair Trading about the deal.
Absolute Radio boss Donnach O'Driscoll said that restrictions are still in place for ITV's advertising sales following the merger of Carlton and Granada in 2002, and so the same approach should be applied to Global and GMG Radio.
"Absolute Radio opposes the proposed sale of GMG Radio to Global in its current form," he said in a blog post.
"The combined group would have a market share of more than 50% of total commercial radio listening. In key 'Out of London' urban areas their market share would rise even higher such as Manchester 56%, Glasgow 53%, Birmingham 58% and Cardiff 61%.
"In 2011 ITV's share of Commercial TV viewing was less than 40% and a CRR [Competition Rights Renewal] mechanism remains in place to prevent market dominance abuse in TV advertising trading.
"This deal would therefore enable Global Radio to distort fair and effective competition and provide no benefit to commercial radio's listeners or advertisers."
A Bauer Media spokesman said that the GMG Radio deal would "permanently lessen" choice for radio advertisers.
"Bauer Media believes the merger of Global and GMG Radio will be fully investigated by the competition authorities as this combination will increase the dominant market position that Global already has in local and national markets and permanently lessen choice for radio advertisers," he said.
"We have no further comment to make at this time."