The merger was first proposed last September after a period of speculation over the future of Deutsche Telekom's T-Mobile brand in the UK, and is now expected to close during the spring.
Gervais Pellisier, deputy CEO and CFO of Orange owner France Télécom, said the way was now clear to "create a strong operator and to offer high-quality network and services to our customers" and pledged to "generate significant value for our shareholders".
The new business is expected to post pro forma 2009 sales of £7.57bn and EBITDA of £1.38bn.
The joint network will be the largest in the UK by some margin, a point which attracted the attention of the Office of Fair Trading last month.
However, mobility experts have long maintained that networks in other European countries control far larger tranches of the market than Orange and T-Mobile will.
The board of the new JV will be led by representatives of both firms, with Deutsche Telekom CFO Tim Höttges and France Télécom's Gervais Pellissier holding the post of non-executive chairman on a rotating basis.
The management team will be led by Orange UK CEO Tom Alexander as chief executive, and T-Mobile UK CEO Richard Moat as COO.
Both France Télécom and Deutsche Telekom have committed to maintain separate branding in the UK until 2012.