Both the mayor of London Boris Johnson and the justice secretary Michael Gove are "leaning towards the exit", two well-placed sources have told me.

Or to put it another way, they are both on the verge of declaring they will campaign to leave the EU, in opposition to the prime minister.

This would be explosive.

They are huge political beasts. So would be great assets to the leave EU cause.

But it is their reasons which are perhaps most damaging to the prime minister - which is that both are underwhelmed by Downing Street's plan for new legislation to assert the primacy of parliament over EU law (along the model of Germany's constitutional court).

If the justice secretary has profound doubts that the legislation would be effective, then its credibility would be damaged at birth.

"Boris saw the prime minister today and was unpersuaded. He and Michael are in touch with each other and have shared their concerns with each other," said a source.

Another source said: "I cannot see Boris turning back. His price is now too high (for the prime minister)".

There are two other points of note tonight.

First my sense from those involved in the UK-EU talks is that the prime minister is likely to give way on his initial demand that child benefit restrictions should apply to all migrants - and will instead agree to the demands of critics from the Visegrad four of Poland, Hungary, Czech Republic and Slovakia that "indexing" of child benefit (payments at a lower rate) should apply only to future migrants.

This will be seen by British eurosceptics as a betrayal by the prime minister. But I understand he will only give ground on this if he gets tougher action on other benefits paid to migrants - so perhaps a lengthening in the number of years the UK could apply the new brake on in-work benefits.

Finally I have learned that Downing Street has made provision for the historic cabinet meeting to approve the new EU settlement to take place early on Saturday morning - though I am told the prime minister still has a hope the meeting will be Friday afternoon.