I am applying for Internships in London for Summer 2006 and need to know 30-40
reputable employers in the Advertising, Banking & Finance, IT,
Management, Consultancy and/or Retail sectors whose application
deadlines are still open.

Internships meaning placements for students currently in University
and for the summer months (8-12 weeks in length).

Reputable employers meaning those with a good standing in their
respective sector and good to put on my CV!

Where are you studying?  Does your university have an office that can help you?
If you are in England, do recruiters from companies come to the
university to interview?  If so, the office that arranges interviews
could help you with a list of companies that think well of the
university and its students.

I should learn to search before commenting.  Look at this:

http://www.e4s.co.uk/docs/internships.htm

Good luck!