The two giants — currently the UK's third and fourth largest operators — will have 28.4million customers between them and claim a huge 37 per cent market share.
France Telecom-owned Orange and Deutsche Telekom's T-Mobile are in exclusive talks over the merger.
If it happens, they have promised better coverage and improved customer services.
But they warned there would be job losses among their combined 19,000 UK workforce.
There would be "efficiencies" across both businesses, including networks and IT, marketing and advertising, according to an Orange spokeswoman.
The deal, expected to be signed by the end of next month and completed in 2010, comes as mobile phone operators struggle in a highly competitive — and saturated — UK market.
'Benefits'
Their tie-up will whittle down the number of major players further, while also relegating market leaders O2, owned by Spain's Telefonica, and Vodafone to second and third place.
Customers were told the merger would "bring substantial benefits".
But they were told they will not see any change in branding for the first 18 months, with Orange and T-Mobile remaining separate while branding alternatives for the joint venture are reviewed.
France Telecom's chief financial officer Gervais Pellissier said: "By combining our operations in the UK, we anticipate the long-awaited consolidation in one of Europe's most competitive markets, thereby creating a well positioned player.
"This will reinforce fair competition and will provide strong benefits for our customers through improved coverage, quality of service and an enhanced capacity to develop new services and technologies."
The pair are now in "exclusive" talks over the 50/50 joint venture, which comes after Deutsche Telekom considered an outright sale of its embattled T-Mobile business in the UK.
It is thought that Vodafone and O2 were also interested in a deal.
T-Mobile was put up for sale earlier this year by its German parent after seeing three quarters of negative sales growth as it struggled in a fiercely competitive market.
New management were installed in May, headed by Richard Moat, in a bid to revive the business.
But a merger with Orange will help both firms take on the might of Vodafone and O2, with annual sales of £7.7billion and earnings of £1.7billion, based on pro forma 2008 figures.
They also hope to make annual cost base savings of more than £445million from 2014.
The merged company will be headed by Tom Alexander, chief executive of Orange UK, who will take on the same role, with Mr Moat as chief operating officer.
Orange employs 12,500 in the UK, while T-Mobile has a 6,500-strong UK workforce.
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