The move is also part of the tax authority’s drive to force people to use online services rather than calling helplines.

In a letter to the Treasury Committee, HMRC deputy chief executive Angela MacDonald said: ‘The decision to pilot a seasonal telephony model for SA was taken based on the need to improve overall customer service levels.

‘This is also set against the context of the challenging level of efficiencies required by our Spending Review 2021 settlement. This challenge has been subject to further pressure from inflation and policy decisions, directly impacting the number of taxpayers, and the number with more complex affairs, that we have to serve.

‘We recognise that this pilot will test a significant change in our services and will need careful monitoring, but our analysis shows that we should be able to handle the vast majority of SA queries over the summer through our digital service, backed up with a webchat adviser service.

‘We will evaluate how our customers respond to this digital shift and use this experience to improve SA and other services in the future.’

However, the Treasury Committee was critical of the level of information provided in the response, which failed to address MPs’ concerns flagged in a hearing with HMRC officials last month. It also criticised the abrupt nature of the closure announcement, which gave taxpayers less than a week’s notice. There was also no consultation about the decision.

Commenting on the correspondence, Harriett Baldwin MP, chair of the Treasury Committee, said: ‘I am disappointed by the lack of detail and transparency displayed by the leadership of HMRC in response to my questions on the summer closure of an important taxpayer helpline.

‘There is clearly a lack of clarity over the impact this decision will have on taxpayers. This simply isn’t good enough.

‘These decisions should not be taken in haste and with no consultation, and as a Committee, we will be keeping a close eye on developments in this area.’

McDonald strongly rejected claims by the Treasury Committee that the reason for the closure was down to a lack of resources or home workers.

‘I can reassure the Committee that this pilot is not related to our flexible working policies in any way. No fewer people will be employed answering enquiries and processing customers’ tax affairs, no staff will be working fewer hours, and nobody will be doing less. The staff who would have been on this phone line will be working in other customer service roles during the pilot.’

The closure will see around 350 staff transferred to other helplines to support more urgent queries during the three-month pilot period.

HMRC was adamant that the closure was due to low demand over the summer but did not rule out the possibility that the closure could become more permanent.

‘Our aim is to offer the best possible customer experience at the most efficient cost to the taxpayer. Data collected during the pilot period will allow us to assess how the SA service is used, any impacts on other lines and services, customer feedback, and customer behaviour. This will include monitoring any additional contact through other HMRC telephone lines and via post.’

HMRC stressed that up to two-thirds of calls to the self assessment helpline could be resolved online, either by consulting online guidance, using the digital assistant or by going to a taxpayer’s personal tax account or the HMRC app.

It also said that the number of calls was down 50% during the summer months compared with the returns season from January to April.

‘SA queries during the summer tend to be less complex than those received later in the year and relatively non-urgent compared with calls to some of our other helplines,’ McDonald said.

However, online services need to be improved before taxpayers can rely on the information to make complex tax decisions.

‘We are working hard at extending and improving our online services, which will enable customers to self-serve at times and in ways that are convenient for them,’ McDonald said.

Use of online services ‘enables us to be more efficient and productive with our available resources, but also ensures that phone services are available for customers with the most complex queries or who are unable to access our digital services’.

The correspondence comes in response to a series of questions from the Committee chair on HMRC’s decision, including the impact on taxpayers, whether the helpline will be reopened should the detriment to taxpayers be greater than expected, and if the closure is related to HMRC’s homeworking policy or staffing issues.