Sir Menzies Campbell said three MPs from each party who have never been in Government should sit down to negotiate an amicable end to the alliance.
But in an interview, the outgoing North East Fife MP conceded he had yet to convince anyone of the merits of his proposal.
He said: “The ministers will have to keep going to the very end. Why? Because the country has to be governed.
“But I think we should accept that the point’s going to come at which we may be together governmentally, but politically we’re going to start – well it’s started with differentiation – moving away from each other.
“And we should do that without recrimination or acrimony or intimidation or anything of that kind.”