HMRC said its network – including those which businesses use for paying tax – will be affected from close of play on Thursday, 5 April, until next Tuesday, or in some cases next Wednesday.
Self-assessment and PAYE agents and customers will not be able to file tax returns for the new tax year until the services have been upgraded, although VAT customers will be able to file online as usual until midnight on Saturday, 7 April – the official deadline for payment.
HMRC has though said if, for any unforeseen reason this is prevented because of the changes, it would count as a sufficient excuse for late payment.
But the Forum's Chief Executive Phil Orford said it was the wrong time to schedule the work: "Many firms will find it hard to understand why HMRC has chosen to do a major upgrade to its IT systems at such an important time of the year in the tax calendar.
"With regards VAT returns, if this shutdown does lead to problems and customers find they cannot file their returns on time, we would hope HMRC has put in place a plan and is ready to deal with the fallout quickly and efficiently for those affected.
"We know from our members' own experiences in the past, however, that once a fine has been issued because of HMRC mistakes, it can take months and months of wrangling to sort out and for firms to be reimbursed.
"While HMRC says this is a routine shutdown and has been planned, it will come as little solace for those businesses which are affected. The fact it was only announced last week as well shows little understanding for the needs of businesses."
Forum members have also criticised HMRC's short notice and timing of the network shutdown. Rachel Andrews, who runs Andrews Computers in Hemel Hempstead, said: "I make my living teaching people how to do this and supporting them through the process, and my diary has been booked with client appointments since January. HMRC announced this just last week. Where is the commitment from them to give us timely warning and fair notice?
"This simply shows a spectacular lack of insight on their part."
She added: "I think this is really unhelpful for business as it narrows the window when the submissions can be done, and means that it puts even more pressure on firms when they least need it.
"A large number of small businesses rely on a third party or their accountant to do this submission for them. The professionals can't work 24/7, though, and need to schedule clients in over the period from the end of March to the end of April.
"To schedule maintenance now just shows a lack of real world appreciation. HMRC needs to stop this stupidity and make a commitment to business owners to treat them fairly, and give fair warning of events like this, and to not close over busy periods.
"Mid-month, over the summer is fine; there are no major deadlines after 19 July for P11D. Alternatively, why not close it in February when there are no major deadlines?
"They wouldn't close it at the end of January because of self-assessment, so why April, when it's payroll year-end?"
The Forum of Private Business has criticised HMRC when, with just days to go before the end of the current tax year, the department announced plans to shut down its entire IT systems for nearly a week.