Tax inspectors have been accused of being so incompetent they pose a threat to revenue collection, just months before a controversial new Scottish levy is introduced.

Scots will have to pay the new Scottish Rate of Income Tax (SRIT) from next April.

Research suggests that many workers were still unaware of the newly-created tax and whether or not it would affect them.

Now MPs on the influential Commons Public Accounts Committee warn that half of all telephone calls to HMRC contact centres, designed to help taxpayers with queries, currently go unanswered.

The problem is having an "adverse impact" on the collection of taxes from people keen to pay what they owe, MPs say in a new report.

The committee previously described HMRC's record on the issue as "abysmal".

But it found that the numbers of calls picked up had actually fallen in recent months, from 72.5 per cent in 2014-15 to just 50 per cent in the first half of this year.

Meg Hillier, the Labour MP who chairs the committee, said HMRC's customer service levels had now declined to the point where it represented a "genuine threat to tax collection".

"It beggars belief that ... the taxman seems incapable of running a satisfactory service for people trying to pay their fair share," she said.

In a wide-ranging report, her committee also accused HMRC of failing to prosecute wealthy tax evaders who hide their money overseas.

There have been just 11 prosecutions for offshore tax evasion over the the past five years, resulting in sentences collectively totalling just 15 years in prison.

Only one of the cases related to an individual on the "Falciani list" - leaked by a whistleblower at HSBC's private Swiss bank - which identified around 3,600 UK individuals potentially engaged in tax evasion.

The committee described the figures as "woefully inadequate" and warned that they did not represent a credible deterrent to those who wanted to break the law.

The report accuses HMRC of ignoring previous warnings from the committee that it needed to show that it could deal "robustly" with individuals and firms.

MPs also attacked what they said was HMRC's continued refusal to report on the scale of "aggressive tax avoidance".

Under the new SRIT income tax rates set by Westminster will be cut by 10p in the pound in Scotland.

Scottish ministers will then face a choice about how to generate income.

The SRIT could be set at 10p, meaning the amount workers pay overall would remain the same, or cut it, even to zero.

The system has been created to give Scottish ministers responsibility for raising more of the money that they spend.

Scottish Finance Secretary John Swinney has yet to say what next year's rate will be.

HMRC is due to contact workers over the next few months to confirm that they will be be classed as Scottish taxpayers under the new rules.

Tax collectors say that the simple rule is that status will be decided by place of residence rather than where people work.

But HMRC progress reports reveal that "red alert" areas of concern include identifying Scottish income taxpayers.