The PSA is too political, according to the advertisement approval agency Clearcast, which says the 60-second spot could violate the 2003 Communications Act.
"These rules ensure that adverts aren't being broadcast by bodies whose objects are wholly or mainly political," a Clearcast spokeswoman told the BBC.
"One appears to be caught by this rule as they state that part of their raison d'etre is to pressure political leaders. It also appears that a number of the claims made in the version of the ad that we have seen are directed towards a political end, which is again against the rules."
A spokesman for One.org of course insists that their cause is purely humanitarian and not at all political.
"We recognize the purpose of the broadcasting code is to keep political propaganda off British television," said Adrian Lovett, One's Europe director.
"But our ad highlights the desperate plight of 750,000 people in east Africa who, the UN warns, could die before the end of the year. Unless we keep the spotlight on this crisis and the need for urgent action, those people will be forgotten. Who can object to that message? We are challenging this decision and hope the broadcasters will reconsider."
In the meantime, any station that airs the ad could be subject to a fine or even lose its license.
We can imagine what Jessica Alba thinks of the decision. The actress tweeted just Wednesday: "30,000 children dead in 3 months in Somalia? Where's the outrage? Watch our 'F WORD' PSA & help."