ALISTAIR Carmichael has warned Labour it could be punished by Scottish voters at the 2015 general election if it tried to put a brake on more powers for Holyrood as he urged Nicola Sturgeon to "keep your word" and rule out a second referendum on independence.

Speaking at the Liberal Democrats' conference in Glasgow, the Scottish Secretary cautioned that any hindrance to more powers would help the Nationalists keep independence at the forefront of political debate for years to come.

While the LibDems and the Conservatives support devolving virtually all powers on income tax to the Scottish Parliament, it is thought Labour is deeply split on the issue. The target date for reaching agreement among the UK parties is just weeks away - St Andrew's Day.

Mr Carmichael said: "Anyone who threatens to delay the process will suffer electorally in 2015 and risk putting us in a position where the pledge is seen not to have been honoured and, as a result, would not settle the independence question. The stakes are high. The delivery of the pledge means as much to those who voted Yes as to those who voted No."

He stressed that ensuring the greater powers were delivered meant Yes campaigners had "nowhere else to go" given the referendum was fair, decisive and legal.

In his keynote speech, Willie Rennie, the Scottish LibDem leader, addressed what he called Labour's "fear" about the consequences for the rest of the UK from constitutional changes for Scotland but he stressed: "Now is not the time for timidity here in Scotland or across the UK. The response to the cry from voters is to be bold and ambitious."

Earlier this month, Gordon Brown suggested that handing over control of income tax to Holyrood would help the Tory leadership in its bid to ban Scottish MPs from voting on financial matters at Westminster.

Such a move, which the former Labour prime minister described as a Tory trap, would be "clearly against the material interests of the people of Scotland and put the Union itself at risk". It is thought Ed Balls, the shadow Chancellor, agrees.

Earlier, Mr Carmichael urged Yes campaigners to respect the referendum result, saying: "We can all wave flags, join protests and march if we want to, but wouldn't it be better to set those things aside to accept the will of the Scottish people."

Mr Carmichael claimed Ms Sturgeon, the deputy SNP leader tipped to take over from Alex Salmond, now faced a big test.

He added: "It is disappointing she has declined to rule out pushing for a second referendum in less than three years from now.

There is still time for her to clarify that she will push for no such thing. I hope she will take that opportunity."

The Scottish Secretary, citing the example of Canada, warned that the threat of a second referendum would undermine business. "I don't want the pall of uncertainty to descend once again, obscuring our view of the future," he said. "The Scottish Government should join us in doing everything possible to create certainty and protect jobs, not cast doubt and scare employers away."

And in a direct challenge to the Deputy First Minister, he said: "So, Nicola, take this moment to keep your word, uphold the Edinburgh Agreement and rule out your support for another referendum."

The Deputy First Minister argued that the word "devolution" was inadequate to describe the current process because it suggested powers being handed down to a passive people.

And Ms Sturgeon said it was remarkable 1.6 million Scots voted Yes given the No camp's fear tactics.

The September 18 poll would, she said, "be judged in days to come as the moment which determined that independence was a question of "when, not if".

Writing in periodical Scottish Left Review, she said: "The more responsibilities we can demonstrate Scotland is capable of successfully discharging, the less people will heed the siren voices."