Plans to move HM Revenue and Customs services online could lead to another potentially "disastrous decline" in customer service, MPs have warned.
Customer service staff are being cut by about a third as HMRC expects more people to access services digitally.
But a report by MPs says it is "not convinced" the tax authority will not repeat the "collapse" of 2014-15, when call waiting times tripled.
HMRC says average waits are now five minutes and it aims to get that down.
The report by the Commons public accounts committee notes that HMRC, which has to save £98m by 2021, expects to employ 16% fewer staff and close 137 offices. Those remaining will be largely working in 13 regional centres.
The tax authority is aiming to make most processes automated and have more staff focusing on specialist work to crack down on tax avoidance and evasion.
"HMRC expects that, as it introduces more online services, a channel shift will occur resulting in a reduction in the demand for phone calls as more people will be able to meet their needs through online services," the MPs report said.
But the committee noted the "collapse" of customer services in 2014-15 after 5,600 staff were made redundant.
It highlighted that during an 18-month period, call waiting times tripled and some customers were kept on hold for up to an hour.
The MPs said there was an "enormous challenge" ahead for HMRC, while it also restructures its business, relocates many of its staff and moves to a new IT contract.
Having already lost its chief digital and information officer, any more departures of key staff would be damaging, they warned.
The committee said: "HMRC is staking a great deal on the success of its plans to digitise the tax system, but once again it lacks an adequate plan if demand for its call centres does not reduce as quickly as it hopes."
Committee chairwoman Meg Hillier said: "The lack of a convincing fall-back plan to safeguard service as HMRC undergoes significant change remains a looming threat to its ability to collect tax from individuals simply trying to pay their fair share.
"By the spring we will expect to see evidence that HMRC has agreed measures with the Treasury to ensure it is not left playing catch-up at taxpayers' expense."
The committee also restated its concern about the "unnecessary hardship and suffering" to tax credit claimants caused by HMRC's failed contract with private firm Concentrix to tackle fraud and error and urged greater transparency about the tax affairs of multinational corporations to ensure they "pay their fair share of tax".
Mark Serwotka, general secretary of the Public and Commercial Services union, said the report showed staff cuts had damaged services and there was now an "overwhelming case" for HMRC to stop plans to "close 90% of its UK offices and axe thousands more employees".
A spokesman for HMRC said: "We now consistently answer 90% of calls first time, in an average of less than five minutes.
"We have invested heavily in customer services, recruiting more than 3,000 new staff who are also available outside normal office hours when many of our customers choose to call us.
"This is alongside a new range of popular digital channels for customers to get the information and support they need without having to pick up a phone or pen."
The department added that it had "led the way" on global tax transparency and tax avoidance by multinationals and its efforts to crack down on tax avoidance, evasion and fraud had brought in £26.6bn over the last year.