JIM Murphy has played down suggestions Labour should back the devolution of 100% of income tax to Holyrood.
The MP, one of the frontrunners for the Scottish Labour leadership, said it would not make sense to give up one of the "real strengths" of the Union.
His comments follow reports that Labour MSPs and MPs have been sounded out about the potential beefing up of Labour's position on extra tax powers.
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Today a respected economic think-tank also warns that handing MSPs full control over income tax could undermine the Union.
The National Institute of Economic and Social Research (NIESR) says the move would deprive Scottish MPs of votes on an issue of central importance to the UK economy, income tax in England, because it would greatly strengthen the case for "English votes for English laws".
Labour is under pressure from both the Conservatives and the Liberal Democrats, who both say they support the full devolution of income tax. However, The Herald understands that opposition to the idea among Labour MPs has hardened in recent weeks.
One MP said: "Even some of those who previously said they did not have a strong opinion one way or the other are now against it."
Labour's draft devolution commission report said the party was minded to devolve all power over income tax. That was later changed to a significant proportion, amid fears of undermining the principle of the pooling and sharing of resources within the UK.
One of Mr Murphy's main rivals for the Scottish Labour leadership, MSP Neil Findlay, has expressed his sympathy for the call for full income tax powers.
Yesterday Mr Murphy said: "One of the important things about a tax system - and we saw this in the financial crisis and crash of 2008 - is how across the four nations you have a tax system that supports one another, that spreads wealth around the United Kingdom.
"Now, when Scotland has voted so strongly - with over two million people voting No to stay in the United Kingdom - it wouldn't make sense to give up voluntarily one of the United Kingdom's real strengths. A devolved tax system, yes - but while retaining aspects of a UK system ... sharing and pooling and that solidarity that comes from being part of something bigger."
Speaking on the John Pienaar programme on the BBC, he said Labour's two-year devolution commission had come up with "ambitious" proposals. And, in an apparent nod to Mr Findlay, he said that "all three" of the candidates in the race "have decided not to rewrite two years of work".