SCOTTISH Labour divisions on more tax powers for Holyrood have deepened after it emerged that the next chairman of the party's MPs blasted the plans as "weak" and "not serious politics".

Michael McCann also said MPs "weren't consulted" about the devolution commission proposal to give the Scottish Parliament control of income tax and insisted there were "serious questions" about how policy is made in Scotland.

However, senior party insiders slapped down McCann, saying that the MP was "out of touch" and had "contempt" for Holyrood.

Scottish Labour leader Johann Lamont announced the creation of the commission in 2012 to review devolution and the Parliament's powers.

An interim report stated last year there was a "strong case" for devolving income tax, as well as air passenger duty and vehicle excise duty.

The final report is expected to echo most of the conclusions and be approved at Scottish Labour's conference in Perth later this month.

However, some MPs are unhappy with the plan.

The Sunday Herald can reveal that McCann, the MP for East Kilbride, Strathaven and Lesmahagow, outlined his concerns in a stinging letter to local Labour members in his constituency last year.

In the letter, a copy of which has been obtained by this newspaper, he said of the income tax plan: "The announcement came as a surprise to MPs because we weren't consulted, despite the fact that income tax is a reserved power held by the UK Parliament.

"Leaving aside the way the announcement was made and handled, serious questions have to be asked about how we make policy in Scotland. It appears that every time we contemplate policy changes the Scottish Labour Party feels the need to reach into the reserved powers bag, as if it contains the panacea for all ills; let me tell you it doesn't."

McCann, who is expected to become the new chairman of the parliamentary group of Scottish Labour MPs, also claimed the tax proposal undermined the case against independence.

He said: "In the independence referendum campaign we argue that we are better when we pull our resources as a United Kingdom together; yet by proposing to devolve income tax we defeat our own argument."

On the possibility of devolving air passenger duty and vehicle excise duty, he said: "Serious questions arise about these proposals. What's the rationale for the devolution of such powers?

"What will the impact be on the Barnett Formula? Why devolve these taxes and not others?"

In another swipe, he said: "All too often these policy suggestions appear to make us look more Scottish than the Nationalists. That's a weak position and it's not serious politics because we are a UK party."

He added that the Labour consultation on the tax plans had to be "UK-wide".

This newspaper has also obtained a copy of McCann's submission to the review of Scottish Labour following the 2011 Holyrood election rout.

The review led to the creation of a new post of Scottish Labour leader and more autonomy for the party north of the Border.

However, McCann said he was a member of the "British Labour Party".

He added: "The component parts of the BLP have always had their separate identities but they have not been autonomous. Neither should they be."

And he said: "If we widen the scope and role of a Scottish leader and deputy Scottish leader we undermine the British Labour Party."

A party insider criticised the MP, saying: "Michael McCann treats the Scottish Parliament with contempt and is not the brightest politically."

Another party source said: "He is so out of touch."

SNP MSP Annabelle Ewing said: "The splits and rebellion in the Labour Party are spreading out of control, as Johann Lamont struggles to hold on to effective leadership of her party. We have seen briefing and counter-briefing from Labour politicians at war with each other while MPs are preparing to boycott Labour's Scottish conference."

A Scottish Labour spokesperson said: "We believe the people of Scotland want to see devolution strengthened and we look forward to unveiling our proposals later this month."