Denton Hall is acting for Energis on its £352m acquisition of Netherlands-based EnerTel in a move to create one of the largest telecoms groups in Europe.
Niche City firm Rakisons is also working on the sale, advising its long-term US client WorldPort Communications on the deal.
The US group is the holding company for Dutch subsidiary EnerTel, the country's largest alternative telecoms provider.
The deal is the one of the largest ever completed by the seven-partner firm.
Nicholas Higham, telecoms partner at Denton Hall is leading a group of nine lawyers on behalf of Energis, including corporate partner Steven Goodman and tax partner Judith Harger.
Rakisons' six-man team is led by corporate finance partner Michael Thomson with support from telecoms partner Daniel Preiskel and Simon Vivian, a partner in the corporate and commercial department.
WorldPort instructed Rakisons just over a week before the deal was completed.
Thomson confirms the timescale was rushed for a small practice, but he adds: “The good thing is that both firms worked positively together to get the right result.
“It is crucial because if the law firms do not get on then time has been wasted.”
The deal is expected to be closed by early 2000, depending on WorldPort shareholders and the Securities and Exchange Commission.
Energis was the telecoms arm of the National Grid Group until it was floated at the end of 1997.