More than 400 Members of the European Parliament registered support for a declaration seeking a review of the regulations.
The majority obtained during the campaign meant that the declaration was officially adopted by the European Parliament.
"We are one step closer to ending these cruel and unnecessary journeys," World Horse Welfare said.
"It is essential that we keep up momentum by continuing to highlight this issue with local politicians as well as the European Commission."
The charity says the unnecessary long-distance hauling of horses across Europe for slaughter causes needless suffering to the animals and must be stopped.
It has labelled the hauling of 100,000 horses across Europe each year to abattoirs the single biggest abuse of horses in Europe.
"The terrible conditions endured during these long journeys mean that they suffer from disease, dehydration, exhaustion and injury - all for a needless trade. The infrastructure already exists to make slaughter at source and a carcase trade possible."
It urged supporters to continue to raise awareness about the campaign and write to their European Parliament members thanking those who supported the declaration and urging them to get more involved in the campaign.
It suggested they also write to the European Commissioner for Health and Consumer Policy, John Dalli. He is responsible for any amendments made to the legislation covering the transport of horses to slaughter (Council Regulation (EC) No 1/2005).
"Ask him to introduce short, finite journey limits for horses being transported in Europe. Get John Dalli's contact details," the charity said.
It said backers could also send a postcard (link below for more information) to the president of the European Commission, José Manuel Barroso, asking him to stop the practice.