More than two million filings had been made online by 6 January, a 25% increase on the same time last year. Advisers and individuals filed three million online last year, itself a 40% increase on 2004-05 filings.

However, the top six firms have still not fully embraced online filing, due to the complexity of some of the returns they file. Deloitte has filed none online and is just
using HM Revenue & Customs’ test submissions facility, it said.

PricewaterhouseCoopers is filing ‘some’ returns online. ‘While we are using the online system to file some tax returns, we are holding off doing all returns on the system until we are satisfied that it is ready to cope with the more complex cases,’ Clive Mackintosh, head of private client at PwC said.

KPMG is likely to file one-third of its clients online, while BDO has filed from one major regional office.

Grant Thornton will file a ‘significant proportion’ of its 17,000 clients’ returns online, said Francesca Lagerberg, head of its national tax office. ‘We spent a lot of time this year ironing out problems. HMRC have been incredibly helpful,’ she said.

Ernst & Young, like Grant Thornton, was more bullish about online filing. The firm filed a ‘very high percentage’ of its returns online.

‘We attempt to do it online unless there’s a major problem,’ said a spokeswoman.

The firms are expected to file the vast majority of clients’ returns online for the next tax year, as the paper return deadline is brought forward to 31 October 2008.

HMRC will be awaiting the outcome of the 31 January deadline with some trepidation. It is possible that the civil service unions will vote for another strike on that day, which could be a logistical nightmare for the taxman, although last year’s strike proved manageable.

With the ongoing concerns over its data governance, it will hope its normally robust returns systems hold firm, as it does not want to damage public confidence in the department even further or put off the major firms from filing online next year.

The taxman revealed that nearly 500 tax returns were filed online on Christmas day, compared with 84 a year earlier.