David Cameron's EU renegotiation package has come under fire before being formally unveiled, with London Mayor Boris Johnson voicing "doubts" about a key proposal and saying he wants to see "more progress".
A draft deal aimed at meeting the Prime Minister's reform demands will be published by European Council president Donald Tusk shortly, including new powers for national parliaments to block unwanted proposals from Brussels.
Mr Johnson, who has been mooted in some quarters as a potential leader of the Out campaign in the looming referendum, told LBC radio he did not yet know the "quivering magnitude" of the changes.
But referring to plans for parliaments to be able to gang up and wield a "red card" on EU measures, he said: "I will look at it. We will have to see how it is explained to us, I haven't yet got a firm view on it. I have doubts."
Mr Johnson also raised questions about a proposed "emergency brake" on welfare payments to migrants.
"I think what would be better would be if we had a brake of our own that we were willing to use, and that we were more willing to say 'Look, Britain is an independent sovereign country and we don't agree with this particular bit of legislation or regulation and we want to stop it'. That's what we should be able to do," he said.
Mr Johnson said Mr Cameron had "done a very good job of renegotiating at huge speed a very difficult package of measures".
"Everybody would want to see more progress and let's see where we get," he added. "So far he has been doing a very very good job of getting people to see things his way.
"I think there is much, much more, however, that needs to be done."