"It was mentioned vaguely to me about 10 days ago that there were talks ongoing, but I wasn't aware that any decision had been reached," he told BBC Radio's World at One. "I have since discovered that the European policy committee of our party didn't know about it either."
Mr Taylor also warned that his party's representative on the EU Committee of the Regions, in Brussels, might now have to leave the European People's Party, the group to which the Conservative Party belongs.
"It means that the UUP in Europe, which was a member of the mainstream centre-right grouping is now going to be with a very small group of only 17 members in the European Parliament, and should that group disappear, we're going to be left totally isolated."
Doubts were also raised about two other elements of the package - the influence to be wielded by Sir James and his UK Referendum Party, and the money that was reported to be in it for the Ulster Unionists.
Following speculation that Sir James would pay the Ulster Unionists up to pounds 250,000 for the deal, the financial edge appeared much less clear- cut.
Sir James said: "We wanted to inform and create a debate, and obtain a referendum, obviously for all the people of the UK. Therefore, we had allocated a certain amount of promotional funds for Northern Ireland." Asked if those funds would go directly to the UUP, he said: "No. They will go from us directly to the cause of obtaining a referendum."
Mr Trimble said in Belfast that the arrangement was European, and would give Sir James no influence at Westminster. "There is no linkage between us and the Referendum Party," he said.
Given the importance of the nine-strong Unionist vote in the Commons, government whips will be pleased with the reassurance that it would be business as usual.
Donald Macintyre, page 11