Members of Donald Trump's election team met a Russian lawyer in June 2016 "to get dirt" on rival Hillary Clinton.
But British publicist Rob Goldstone told the BBC the material was "very generic" and he was highly embarrassed.
A special counsel is looking at alleged Russian links with the Trump team.
President Trump has denied any collusion, repeatedly calling the investigation a "witch hunt".
Mr Goldstone also played a pivotal role in Mr Trump's visit to Moscow for the 2013 Miss Universe Pageant, after which an unsubstantiated allegation emerged - denied by the president - that he had been filmed at a hotel with prostitutes.
The Trump Tower event
Rob Goldstone was acting as publicist for Azerbaijani-Russian singer Emin Agalarov when he sent the now infamous email to the Trump team on behalf of his client.
"What it said was that my client in Russia had called me, and an attorney had some potentially damaging information about illegal Russian funding to the Democrats that could incriminate Hillary Clinton," Mr Goldstone told the BBC's Nick Bryant.
The response of the president's son, Donald Trump Jr, Mr Goldstone says, was: "If it's what you say it is, I love it."
Mr Goldstone says that at the meeting Kremlin-linked lawyer Natalia Veselnitskaya did present material but it was "very generic dirt". He was highly embarrassed, our correspondent says.
Asked about the significance of the meeting, Mr Goldstone said: "I think you'd be hard pushed to believe that the meeting in itself was evidence of any form of collusion."
But he regrets sending the email setting it up.
"Sitting here today, with everything I've learned over the last 12 months, I would write that email, I would read that email and I would go, 'no, no, no' and I would hit delete."
The Mueller inquiry
Special Counsel Robert Mueller is heading an investigation looking into whether Russia colluded with the Trump team to help him win.
Whether the Trump Tower meeting constituted a breach of US campaign rules is part of the inquiry and Mr Goldstone has been interviewed by investigators.
He says he spent hours with Mueller investigators, who he says were interested in him because he was the only independent voice at the meeting - the "bizarre character in the middle".
But he says they were more interested in the "bigger issue" - "the relationship between Donald Trump and the Agalarovs, and the Agalarovs and the Kremlin and the relationship, should there be one, between Russia and Donald Trump".
Emil Agalarov's father, Aris, is a Moscow-based property developer who was Mr Trump's business partner in taking the Miss Universe competition to Moscow in 2013 and was also working to partner Mr Trump in bringing Trump Tower to Russia, a project that never materialised.
The Miss Universe pageant in Moscow
The unsubstantiated claim that President Trump was filmed with prostitutes in Moscow emerged in the Steele Dossier.
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