How to read Microsoft Excel format (.xls) data file by R?

There seem direct way to read .xls format file (see http://maths.newcastle.edu.au/~rking/R/help/00b/2519.html).

However, there some ways to indirectly read .xls file. For example, you can save the .xls file into .csv (comma separated value) format. Then use R's function read.csv to read it.

The reason to save .xls file to .csv file is that usually there are some columns in .xls file which are strings containing white spaces. So if to save .xls file to white space delimited or TAB delimited, then it is still difficult to read the file into R.