Three reasons why British failed to stop the Japanese attack on Singapore

Main reasons why British failed to stop the Japanese attack on Singapore was because the British far out-numbered the Japanese and Japanese had air superiority and they are much smarter than the British.
The Japanese make a fake attack, and all those fake attacks , they wanted to British to think that they would be attacking from the Northeast. By thinking that way , the British would move their defence supplies from the Northwest to the Northeast.

Secondly, the British didn't think Singapore could be taken from the north, the land side of Singapore.
They thought the Malayan jungle would stop any flanking movements or would severely hamper advances by infantry. But some of the Japanese bicycles from the Malayan people and moved quickly down the Malayan peninsula. And so, the British were not prepared for rapid the advance of the Japanese.

Lastly, the British did not provide the defenders of Singapore with tanks or other heavy armor, believing
 they couldn't be used in the Malayan jungle and they would not be needed if Singapore was attacked from
the sea side, but instead , the Japanese went in by the open backdoor. The Japanese entering for attack was different from the place assumed by the British.

Informations adapted from Fortress Singapore

These information above , were reliable and was supported by http://www.historyproject-2012.blogspot.sg/

From the blog , it says that the Japanese attacked Pulau Ubin and made a intense bombings at Changi, wanting the British to think that the Japanese would attack from the northest. The British moved their defence supplies from the northwest to the northeast , which they predict that would be where the Japanese would attack from. However , it was all planned by the Japanese, and the Japanese went to attack from the northwest, which was one of the reason why the British failed to defence Singapore. Also, the British thought that Singapore could not be attacked from the land, as the physical environment of a jungle was not an ideal territory for the soldiers to move in and attack.

Thus , both sources shows that the Japanese made fake attacks to make the British prediction wrong, and that was why the British failed to protect Singapore. Secondly, both of these sources also mentioned that the British assumption was wrong, they thought that their enemy's soldiers would not attack from the jungles. Hence , these informations are most likely to be true.