In the summer, in a document published on the government’s contracts to let service, the department asked for expressions of interest from research organisations for the study, with funding of up to £80,000.

HMRC said it anticipated that the research will have two stages: a pilot stage and a main stage, with a review point in the middle.

In its project description, HMRC says the pilot stage will test out the key elements of the main study including the methodology (response and levels of engagement with participants; eliciting honest and meaningful responses from the participants); testing ideas to improve tax compliance among this group; and feasibility of expanding the pilot into a full scale research project.

The aim of the main stage of the research is to explore the factors that influence tax compliance behaviour among wealthy individuals in depth, and to research effective means of improving it.

HMRC said the wealthy group includes high net worth (HNW) and affluent individuals. The research aims to engage directly with both of them, or failing that, with their tax agents.

In its description of the contract, HMRC said: ‘This is an increasingly important area of work for the department, and an opportunity to conduct innovative research that will feed into the development of new policies.

‘We recognise that this group may be challenging to engage in compliance research but at the same time the project offers plenty of opportunities for trying new approaches; in addition, we have managed to successfully carry out research with other hard to reach populations in the past, and we could provide a sample of the wealthy population as a sampling frame.’

A HMRC spokesman added: ‘This is social research to help with our understanding of the underlying attitudes and behaviours driving tax compliance by wealthy Individuals. The UK has one of the lowest levels of unpaid tax in the world and we have secured more than the annual NHS Budget in additional compliance revenues since 2010.’

At the beginning of the year the public accounts committee (PAC) published a report on its inquiry into HMRC’s approach to the very wealthiest tax payers, those with incomes of over £20m whose affairs are handled by a specialist unit, and who are each assigned a 'customer relationship manager' to administer their tax affairs.

PAC expressed ‘alarm’ that HMRC has around one-third of these individuals under enquiry at any one time, and in 2015–16 was investigating cases with a value of around £1.9bn extra tax revenue that might be due.

PAC’s report highlighted ‘the amount of tax paid by this very wealthy group of individuals has actually fallen by £1bn since the unit was set up.’

It called on HMRC to consider what further powers could help it improve its understanding of the very wealthy, and formally evaluate the effectiveness of the high net worth unit.

In particular, PAC wanted HMRC to assess what more it could do to deter very wealthy taxpayers from bending or breaking the law, highlighting changing behaviour that has seen avoidance ‘moving from off the peg marketed tax avoidance schemes to complex bespoke schemes’.

At the time, Meg Hillier, chair of the PAC, said: ‘HMRC's claims about the success of its strategy to deal with the very wealthy just don't stack up.

‘Cosy terms such as “customer relationship manager” and HMRC's reluctance to be open add to the picture of arrangements that, while beyond the reach of ordinary taxpayers, are also ill-suited to the increasingly sophisticated methods the super-rich can use to reduce the tax they pay.

‘HMRC must play a stronger role in identifying tax measures which are not being used as Parliament intended and push harder for reform where the rules are open to abuse.

‘It must be willing to engage in an honest and open assessment of its compliance activity and adapt its approach swiftly, making the case for new powers where it needs them.’

HMRC has already run surveys into agents’ experiences of the high net worth unit. The most recent findings, from a 2015 survey conducted by research firm IFF, showed that 81% of the more than 500 agents polled agreed that they could trust the unit to take a reasonable approach in their dealings with the tax affairs of their clients.

Four-fifths agreed that unit takes a co-operative approach when dealing with any tax concerns it has and the same proportion agreed that the unit had actively sought a co-operative relationship with them (each 79%).

The research found agents appeared to welcome the unit’s more informal initial approach when discussing tax issues and transactions prior to reporting these in the self assessment return. Around a quarter (26%) of agents who had had dealings with the unit reported having discussions about tax issues and transactions with the unit before they needed to be reported, and the vast majority of these (92%) said that this helped them in their work for their clients.

HMRC 2015 survey of agents of high net worth individuals is here.