THE Scottish Government will have to pay the taxman an extra £4million to collect income tax if it sets a different rate from the rest of the UK next year, it has emerged.

HM Revenue and Customs said setting a different rate of income tax in Scotland would encourage high earners to become cross-border tax dodgers by falsifying their main address.

In a briefing for MSPs, the tax authority warned the cost of collecting the new Scottish Rate of Income Tax (SRIT) would rise as "a divergence in rates would increase the amount of compliance activity required".

From April next year, income tax will be cut by 10p in the £1 across the basic, higher and additional bands in Scotland.

The Scottish Government will set its own Scottish Rate of Income Tax and retain the revenue raised from it for its own budget, while receiving a smaller "block grant" from the Treasury.

HMRC issued its warning ahead of meeting of Holyrood's finance committee tomorrow (Weds), when MSPs will discuss preparations for the new SRIT.

It said running costs - borne by the Scottish Government - would be £2million to £2.5million if the rate matches the 10p cut in the UK rates.

However, collection costs would rise to between £5.5million and £6million if a different rate were set.

In addition to the extra cost of chasing cross-border tax dodgers, the taxman said Scots would be "more likely to call HMRC if the Scottish rates were higher than the UK rates".

HMRC said it was "confident" it was ready to implement the SRIT next April but was planning a publicity campaign in December to ensure Scottish taxpayers were correctly identified as such.

The agency is trying to find people who live in Scotland but "who do not appear on our current records with a Scottish address," a task it admitted was "not straightforward".

The original estimate for operating the SRIT was £4.2million per year, on top of set-up costs, including new a new IT system, of £35million.

The SRIT is being introduced as part of powers transferred to Holyrood under the 2012 Scotland Act.

It will be superceded in 2017 when new legislation will give MSPs almost full control over the £11billion income tax raised in Scotland.

Finance Secretary will set a SRIT for the first time in January when he announces his budget for the coming financial year.

The Scottish Government would generate an extra £330million if it set the tax 1p above the UK rate, dwarfing the additional collection cost.

First Minister Nicola Sturgeon has hinted her government will not deviate from UK tax rates next year, having described the SRIT powers as "very limited" and "not progressive".

Charlotte Barbour, of the Institute of Chartered Accountants Scotland, said: "My reading of what HMRC are saying is that it will cost more to collect SRIT if the rate is not 10 per cent because they will have to police it more.

"Increasingly, I think SRIT next year is being seen as a dry run to work out who Scottish taxpayers are before the full Smith powers come in."

HMRC said it would not discuss operational details of how it would pursue cross-border tax dodgers.

But a spokesman confirmed a different tax rates north and south of the border would create an incentive to claim a different main address.

He added: "HMRC has long experience of policing such behaviour in an international tax context."

The Scottish Government declined to comment on HMRC's warning about collection costs.

However, a Scottish Government spokeswoman did say: “The Scottish Government will propose a Scottish rate of income tax as part of its budget setting process for 2016-17 – once the UK Government spells out its own spending plans in the spending review due in November, which will have a direct impact on Scotland’s budget.”