A conference centre has said it would not have taken a booking from Britain First if it had known who was behind it.
Wyboston Lakes in Bedfordshire said the group booked under the name Patriot Merchandise for a meeting.
The resort said it was a short-notice booking and the company did not appear to have any obvious connections to the far-right organisation.
The BBC has contacted Britain First for comment.
The conference on Saturday came days after President Trump retweeted three anti-Muslim videos posted by Britain First's deputy leader Jayda Fransen, prompting criticism from Theresa May amongst others.
Pictures posted online by the group's leader Paul Golding appeared to show dozens of people sitting in a small room at Wyboston Lakes decked with banners declaring 'Taking Our Country Back'.
The centre faced criticism online when news of the conference, the location of which had not been made public by the group, became clear.
In a statement released on Sunday evening, the resort spokesman said: "For the sake of clarity, had we known the nature of the event being held by Britain First, we would not have taken the booking, as the values of our organisation conflicts with theirs in totality.
"We can fully understand the concerns expressed, but we can only refer to our company values and principles, our standing as an award-winning employer and assure everyone that we are deeply offended by their presence on our property."
No staff entered the meeting room during the event and the banners were erected only in the "closed off conference room", the spokesman added.
The £2,500 hire cost of the five-hour event will be donated to charity, Wyboston Lakes said.