Mr Justice Mann ruled that Alfred McAlpine must not use the name without the "Alfred" prefix at its construction business.
The court found that to do so would lead members of the public to confuse the firm of Sir Robert McAlpine with that of Alfred McAlpine. If this occurred, Alfred would be guilty of "passing off", that is, encouraging the perception that it was something that it was not.
In a 48-page judgment, the Mr Justice Mann found in favour of Sir Robert. He said: "I find that passing off has been established." He said that Alfred McAlpine's name change had amounted to misrepresentation and would cause confusion among customers.
Building revealed in January that the case was going to hinge on an agreement made in the 1930s between two McAlpine sons after Sir Robert McAlpine died. The court heard that it was agreed that Alfred would operate in the North-west and Sir Robert in the South. The names were retained to distinguish them.
We will be seeking permission to appeal at a future hearing
Alfred McAlpine statement
During the two-week hearing Ian Grice, chief executive of Alfred, admitted that he had expected the rival firm to be incensed by his decision to adopt the McAlpine brand name.
Sir Robert also claimed that Alfred's allegedly poor payment record increased the threat to its reputation.