Why did the British failed to stop the Japanese?

Before the war, the Japanese set up their intelligence services in Malaya and Singapore to provide information on British defence and readiness. They then planned how to defeat the British in the event of a conflict.
Find out how the Japanese gathered their military intelligence:
The Japanese owned lands in Johor and Singapore and observed the construction of the Singapore Naval Base and coastal defences. The Japanese fishing fleet, which accounted for nearly half of the fish supplied to Singapore, conducted surveys of the Malayan coastal areas. Japanese photographers, who operated studios in many Malayan towns, took pictures of roads, railways and military installations. Some Japanese military officers operated incognito; for example, an army colonel named Tsugunori Jadomatsu worked for six years as a waiter in the British Officers' Club.

*Someone who is incognito is using a false name or wearing a disguise, in order not to be recognised or identified.

The British had underestimated the Japanese. They believed that the Japanese army, airforce and navy were inferior. However, the Japanese proved the British wrong. The Imperial Japanese Army had a large modern airforce and naval fleet. The Japanese 'Zero' fighter planes destroyed about half of the British planes, which were old and outdated. The Japanese soldiers were also better trained in jungle warfare. Besides using tanks, they also used bicycles as their means of transport to move swiftly down Malaya.

The Japanese 'Zero' fighter plane

 The British old and outdated plane.

Japanese soldiers on bicycles.

Information taken from : Marshall Cavendish Education (2007). SINGAPORE FROM SETTLEMENT TO NATION PRE-1819 TO 1971. Singapore. MOE Building 

Military intelligence was adapted from The Japanese Occupation of Malaya, 1941-1945 by Paul H. Kratoska