LBC host Steve Allen breached the broadcasting code with a series of offensive comments about the Portuguese, the regulator has ruled.
Ofcom investigated a complaint that Allen, who hosts the early breakfast show on weekdays on the speech-based radio station, m ade discriminatory comments.
Listeners of his show, on LBC 97.3FM, heard Allen respond to an email from a listener with the remark "he comes from Portugal. Obviously, the further you get abroad, the dumber they become really".
Shortly afterwards, while discussing another, unrelated story, Allen said: "I suspect they are living in Portugal. That's about the saddest place we have found at the moment where they really are a bit thick."
LBC described the reference to Portugal as " irrelevant ", saying that Allen believed his listeners would have understood that he "did not intend to make a serious generalisation about a particular nation".
But Ofcom said the remarks amounted to a series of "repeated, insulting comments which had the potential to cause offence to listeners" and breached the broadcasting code.
It called the remarks a "series of insulting comments towards a listener's level of intelligence and an unwarranted indictment of the intelligence of Portuguese people generally".
An Ofcom spokesman added: "We found repeated comments by presenter Steve Allen about people from Portugal were derogatory and in breach of our rules. They were not justified by the context and were likely to have exceeded listeners' expectations."
Ofcom also assessed but decided not to investigate complaints about EastEnders, Good Morning Britain and Celebrity Big Brother.
Responding to complaints that the deaths of Ronnie and Roxy in Albert Square were unsuitably graphic, it said "the scene was appropriately limited for broadcast before the watershed."
It said that a Celebrity Big Brother task, which saw housemates play "directors" who had to "edit out" Stacy Francis, was "consistent with an established element of this long-running series, and would have been well within viewers' expectations."
After complaints that Susanna Reid's Good Morning Britain co-host Piers Morgan reduced a woman - who billed another mother after her daughter's shoes were damaged on a play date - to tears, Ofcom said "the different approaches of the two interviewers helped to balance any impact on the guest".