Fear of another independence referendum will force David Cameron to follow through on his extra-powers pledge, senior Liberal Democrats believe.

The Prime Minister is expected to be pressed on the issue when he jets into Scotland today in a last-ditch bid to prevent a "Yes" vote.

The trip is expected to be his last before ­Thursday's referendum - amid accusations the Conservative leader is "toxic" in this debate.

Last week the Tories, Liberal Democrats and Labour agreed a hastily-arranged timetable for extra powers, set out by former Prime Minister Gordon Brown. Yesterday, Mr Brown announced that the first Commons debate to kick-start that process has been confirmed for the middle of October.

But there have been ­accusations from the "Yes" campaign that the pledge on extra powers is a bluff designed to prevent Scots backing independence.

There are also significant differences between the three pro-UK parties over the extent of powers to be devolved, especially over income tax.

One Liberal Democrat minister said that Mr Cameron understood the dangers posed by Canadian- style "neverendum".

The Liberal Democrat minister said that the lesson of Canada was that any complacency could see another vote within just a few years.

Asked if the cabinet understood the risks, he said: "David Cameron does. He gets it."

Another Coalition minister predicted that Labour would agree to devolve more than the limited income-tax powers it now backs.

He said: "Labour will move on this. They have to."