HM Revenue & Customs has already accepted a record number of internet self assessment filings, as it prepares for the last critical hours before the 31 January deadline passes.

So far 1.6m SA filings have been made online, a 38% increase on last year's total figure.

An HMRC spokeswoman said that 'everything's working fine' with its online filing system at the moment, and had not experienced any downtime over the last few weeks.

Paymaster general Dawn Primarolo expected one million SA taxpayers to miss tonight's deadline, who will be hit with an automatic £100 fine. That would put the total fines to be issued at around £100m.

Ten million taxpayers are calculated as self assessment-eligible.