I read from wiki that:
Assam (Assamese: অসম, Hindi: आसाम, and also Hindi: असम) is a black tea named after the region of its production, Assam, in India. Assam tea (Assamese: অসমীয়া চাহ, Hindi: असमिया चाय) is manufactured specifically from the plant Camellia sinensis var. assamica (Masters).1 This tea, most of which is grown at or near sea level, is known for its body, briskness, malty flavor, and strong, bright color. Assam teas, or blends containing Assam, are often sold as "breakfast" teas. English Breakfast tea, Irish Breakfast tea, and Scottish Breakfast Tea are common generic names. 
Is it true? I need Assam tea for brewing a good cup of Chai Tea. If they are the same stuff, then I don't need go down to Chungking Mansions' indian store for it. ( I live in Hong Kong ) I can just grap one English Breakfast Tea in supermarket.
However, in this wiki about English Breakfast Tea, the tea leaves could also come from Ceylon and Kenya:
English Breakfast tea is a traditional blend of teas originating from Assam, Ceylon and Kenya. It is one of the most popular blended teas and the most common form of British tea culture. It was initially known simply as Breakfast Tea, and was popularised by Queen Victoria.