Try what Oggy is suggesting but then use ':' instead of '_' with the current Django:

<a href="{% url 'admin:index' %}">link to admin panel</a>

Which django version are you using? If you're using trunk, change your admin urlpatterns from:

(r'^admin/(.*)', admin.site.root)

to:

('^admin/', include(admin.site.urls))

And then you get a named URL pattern called 'admin_index' which you can refer to. See

http://docs.djangoproject.com/en/dev/ref/contrib/admin/#reversing-admin-urls

for more information