The campaign has been created by Mather Communications, the through-the-line advertising agency. Using the Dogs Trust database, the letter will be sent mainly to women aged 45 and over who care deeply for dogs and have supported the charity in the past.
They will be sent an A3-sized poster featuring a number of dogs reading: "Why is it hard for big dogs like us to find love?" It goes on to highlight dogs' stories and highlighting why they need extra help.
There is also a suggested donation list and a reply cad with a pre-paid envelope.
Adrian Burder, marketing director of Dogs Trust, said: "There's a disturbing increase in the number of big dogs being brought to us across all our centres.
"We believe this campaign has all the elements to really touch the hearts of existing supporters."
The campaign was written by Alan Lofthouse, and art directed by Malcolm Neill.
Dogs Trust is the UK's largest dog welfare charity. It cares for over 16,000 dogs every year through its network of 17 rehoming centres across the UK.
