The case concerned a group of nine antennae installed on the roof of Forsythia House, a block of social housing flats close to the London Borough of Lewisham's Telegraph Hill conservation area.
Cornerstone Telecommunications Infrastructure Limited argued that the installation was permitted development and therefore did not require planning permission.
The Town and Country Planning (General Permitted Development) (England) Order 2015 confers permitted development rights on small, building-based, antennae and cell systems.
But the order specifically provides that masts installed on buildings which are - like Forsythia House - less than 15 metres in height, and which are within 20 metres of a highway, are not permitted development.
Lewisham Council accepted Cornerstone's argument that the poles supporting the antennae were not actually masts and that the development was therefore permitted.
But local resident, Nigel Mawbey, who lives within the conservation area, successfully challenged that decision before the High Court last year.
Ruling on Cornerstone's appeal against that decision, Lord Justice Lindblom said the nine antennae are mounted in four groups, one at each corner of a plant room.
Each antenna is supported by a pole which is yoked to one of four central support poles, themselves attached to a tripod mounted on a concrete plinth. The central support poles are just under three metres in height, but reach up to 13.5 metres above ground level.
In defining the word 'mast', Lord Justice Lindblom, who was sitting with Lady Justice King and Lord Justice Holroyde, turned to the Oxford English Dictionary for guidance.
Cornerstone argued that the word denotes "a tall, self-supporting, structure that supports antennae at a height where they can satisfactorily send and receive radio waves and is capable of providing 360 degrees coverage from a single position".
But the judge ruled that, in the telecommunications context, a mast means any upright pole, or lattice-work structure whose function is to support an aerial or antenna.
A pole does not have to be ground-based, or of any particular, height, scale or design, in order to qualify as a mast.
He added: "This was not the approach adopted by the council when it determined that the apparatus installed on the plant room of Forsythia House did not comprise a mast.
"The council's interpretation was, in my view, too narrow and incorrect. This was an error of law, fatal to the council's decision."
Lewisham Council will now, in the light of the court's ruling, have to reconsider its decision that the apparatus is permitted development.
Last week, the government announced that it is to consult on proposals to "simplify planning processes" across England to speed the delivery of 4G and 5G mobile networks.
The move followed the government's publication last year of its Future Telecoms Infrastructure Review, which outlined plans to extend full-fibre broadband coverage across the country by 2033 and for the majority of the population to have access to 5G mobile coverage by 2027.