The court also highlighted as of 'major significance' a 1953 letter from the Johor Acting State Secretary informing the colonial authorities in Singapore that Johor did not claim ownership over Pedra Branca.
The reply made clear that in 1953, Johor understood that it did not have sovereignty over Pedra Branca.
That reply would also have left the authorities in Singapore with no reason to doubt that Britain had sovereignty over the island, the court concluded.
The court thus judged that although Johor had possessed an original title to Pedra Branca, that title subsequently passed to Singapore.
The dispute between Singapore and Malaysia over sovereignty of the island arose in February 1980, when Singapore protested against a 1979 Malaysian map that placed Pedra Branca within its territorial waters.
The court ruled that by that critical date, sovereignty over the island had already passed to Singapore.
'The court concludes that sovereignty over Pedra Branca belongs to Singapore.' As Judge Al-Khasawneh pronounced these words, smiles broke out on the Singapore side of the Peace Palace's Great Hall.
The 16 judges of the ICJ voted 12 to four in Singapore's favour.
On the issue of Middle Rocks, the court said that Malaysia's claim of original title still held, as there had been no activities on Singapore's side which made it pass to Singapore.
While the ICJ ruling has settled sovereignty issues, other issues remain.
For one thing, Singapore and Malaysia have to discuss how to delimit the territorial waters in the Pedra Branca and Middle Rocks area.