The advertising watchdog received a complaint about a television advert for the dog welfare charity in which the voiceover recounted the tale of Bunty and included the line "Dogs Trust never puts a healthy dog down".
The complainant argued that the claim was misleading because she understood the Dogs Trust handed strays to council dog wardens, who might put the animals down if they were not reclaimed by their owners after seven days.
In its response, the charity said it had an "inescapable legal obligation" to notify the local dog warden if a stray was taken to one of its rehoming centres. The dog would then be passed to the local authority, and if it was not reclaimed by its owner after seven days, it could be passed back to the Dogs Trust or to another organisation for rehoming, it said.
The Dogs Trust said the claim that it never put down a healthy dog was true: of the 15,886 dogs it cared for in 2009, 226 - less than 1.5 per cent - either died or were put to sleep on veterinary advice.
The ASA concluded that the claim was not misleading because it considered the fate of dogs received by local authorities to be beyond the control of the Dogs Trust, and because it understood that healthy dogs whose fates were within its control were not put down.
The statement by the ASA said independent research showed that 2.8 per cent of strays handed to local authorities were put down for reasons other than ill health in the year to 31 March 2010.