David Cameron has made no secret of his hope that core cities across England will choose to be governed by directly-elected mayors.

But Ministers have been keen in the past to stress that there would be no favouritism shown towards cities that took the plunge and decided to adopt a new form of governance.

A series of “city deals” are being negotiated with core cities – Liverpool and Manchester have already agreed theirs – which will devolve authority and funding down to local level.

But the deals had nothing to do with whether cities had mayors, so it was claimed.

As we’ve reported in the past, this was never more than a polite fiction, possibly adopted to placate Liberal Democrats who have their doubts about the mayor policy.

Documents published by the Department for Communities and Local Government make it very clear that cities with mayors have more chance of getting what they want than those without.

However, it still marks a major change of approach for Mr Cameron to say clearly and openly that Government will reserve certain privileges for cities with mayors.

They will take part in a “cabinet of mayors”, chaired at first by the Prime Minister himself. And he also said that the Government would provide city mayors with extra powers and extra resources.

Cities that decide to muddle along without a mayor will be out in the cold. It provides a compelling reason to vote “yes” in a referendum, although it will also provide ammunition to anyone claiming the Government is trying to rig the result.

Ministers are always at pains to point out that the decision is for local people to make, and of course they have no control over the outcome of the referendums in Birmingham, Coventry and elsewhere on May 3.

But Mr Cameron has another reason for making his pledge.

By making it clear that mayors will be significant figures, with clout at Number 10, he has satisfied some of the concerns of potential candidates such as Mike Whitby.

Indeed, it’s significant that he used the same speech to stress that a city mayor is a suitable job for someone who has already made a name for themselves on a national stage.

In Birmingham, someone like Liam Byrne, the Labour MP for Hodge Hill and shadow cabinet member who is leading Ed Miliband’s policy review, comes to mind. It would be no surprise if he was to end speculation and finally announce his intention to seek the mayor’s job.

But, as always, it’s important to stress that there may be no mayor. The referendum has not been won yet.

And while David Cameron and Tory colleagues hope Birmingham will be one of the cities voting “yes”, it all comes down to the vote on May 3 when power lies not in the hands of the public.