Can Working Tax Credits be Claimed for Previous Years

[Ted1]

I think I would have been eligible for working tax credits for some previous tax years, but I didn't claim these as I didn't really know about them before.

Is it possible to claim tax credits for previous tax years, or can tax credits only for the 2004-2005 tax year currently be claimed?

If previous tax years can be claimed for, how far back can be claimed for?

[Ted1]

Forgot to mention, I'm self employed.