Kim Hendry, of the PCS, told The Telegraph: “The proposed increase to four days a week in the office, with fewer than four working days’ notice, would have placed unnecessary and unfair pressure on our members.”
She added that the union would “consider all the tools” including industrial action.
The Conservatives argued Labour’s row with the unions showed the tax was falling into disarray.
Alex Burghart, the shadow chancellor of the Duchy of Lancaster, said: “You will hear no complaints from the Conservatives about the Government delaying the introduction of their spiteful and vindictive tax.
“But there is a massive irony in the fact that this policy has been thrown into disarray because of a dispute between the Labour ministers and their union paymasters.”
He added: “This is a massive humiliation for the Government on one of its flagship policies.”
HMRC will rely on mansion tax inspectors to target properties with, for example, views of the countryside and driveways, according to reports last week. Agents from HMRC will also be able to demand entry to people’s properties to assess their value for the levy.
An HMRC spokesman said: “Our flexible working arrangements pose no risk to the delivery of the High Value Council Tax Surcharge. Where there is a clear business need to do so, we can require staff to attend the office more than the standard 60pc.”
He added: “We are looking at what office attendance is needed for these teams to deliver the best service for our customers.”
