The possibility of nationwide industrial action by postal workers could affect the filing of tax returns due by 31 October.
Paper self-assessment returns must be submitted by that date if taxpayers are to avoid a £100 late filing charge.
It is estimated that some 9.2 million paper returns are expected to be filed by post this autumn.
However, the threat of a national strike, which may begin on or after 22 October, could mean that not all returns reach HM Revenue and Customs (HMRC) on time.
A spokesman for HMRC said that, should a strike take place, returns delayed in the post may be liable to a penalty but that taxpayers could appeal against the charge.
He said that HMRC would take the strike into account when assessing appeals: “There is an existing provision for this. If it is posted on 25 October and there is a strike, then that will have been done in good time so the penalty will be reduced to nil.”
