It had wanted to challenge the verdict which was given after the Mayor of London, Boris Johnson, on behalf of Transport for London (TfL) intervened and banned an advert by the charity from London buses in 2012.
The posters would have said 'Not Gay! Ex-Gay, Post-Gay and Proud. Get over it!' in a direct response to a pro-LGBT campaign by Stonewall displayed on buses, that read: "Some people are gay. Get over it!"
Core Issues Trust claimed freedom of expression had been denied and alleged wrongdoing by TfL in how it came to its decision, however a judge has now dismissed the application to appeal claiming the original inquiry was 'satisfactory and appropriate'.
The charity has now been left with legal costs of £100,000.
Commenting on the case, Dr Mike Davidson, Director of Cores Issues Trust, said: "Neither myself nor the Trust is in a position to pay the exorbitant costs of the Queen's Counsel, barristers and solicitors which Transport for London has felt able to fund so freely from the public purse.
"The case has shown clearly that British officialdom has no regard for the rights of ordinary men and women whose identity is "ex-gay" and whose practice is to leave their homosexual past.
"We will not be intimidated or silenced."
Core Issues Trust says it's now seeking legal advice on what its next move is.
Listen to Mike Davidson speaking to Premier's Marcus Jones on the News Hour:
