Refusing (declining) an invitation

Here is a sample letter to refuse an invitation

Dear Sirs,

I am pleased and honoured by the invitation to participate in your conference. I very much
regret to say that it will not be possible for me to participate this time. Between 20th and 30th June 2012. I am giving a series of
lectures at the University of South Essex. It was arranged last January and I cannot change this commitment any more.

Thank you once more for honouring me with your invitation.

With very good wish for the success of the conference,

Yours faithfully

(Sd/-)
Andrew Smith