After the mess that the British Mandate left after realising the serious issues that had been caused by the unorganised mass migration of Jews into Palestine, Britain had officially handed over responsibility of the land to the United Nations to lead negotiations towards a two-state resolution comprising of an Arab state and a Jewish state.
The 1947 UN Partition Plan was then drafted. This plan would have been the only source of "potential" credibility for anyone to claim any legitimate bordering, but it was rejected by the Arabs.
The main reason for this was that it seemed totally unbalanced to provide the Jews with 56% of the land when they only comprised approximately 12% of the population. The following map describes the situation more accurately:
The Arab's disapproval was presented to the UN so that they could go back to the "drawing board" and re-draft another resolution. It was obvious why the Jews would agree to such an advantageous portion of land, but in the manner of speaking with regards to the interests of both parties, a better resolution was obviously required to be reconsidered.
As far as the Partition Plan was concerned, the land of Palestine was still considered disputed territory during the conflict because neither agreed on a final decision, and therefore neither was able to claim authority or to declare independence.
It would be interesting to know who in the Jewish sector actually felt they could claim the land independent to the Jews unilaterally, especially when it was still at large, in dispute.
That would most likely explain the reason why the Arabs were increasingly upset that it was carried out without a just and fair decision being finalised, especially when it was made without a bilateral agreement.
Some Israelis claim that since the land was "up for grabs", the Arabs should have taken their portion and declared their own independence as well, but if they had "officially" declared West Bank and Gaza as their own sovereign states, by implication that would mean that they had agreed to the Jews taking most of the land when their share was grossly unbalanced.
Almost 20 years later, Israel annexed the rest of the land.