By Karen Kiernan
Business reporter, BBC News
The man who fronted adverts for HM Revenue and Customs (HMRC) with the slogan "Tax doesn't have to be taxing" says: "I wish tax was simpler".
In adverts, historian and BBC presenter, Adam Hart-Davis, has urged the public to complete their tax returns on time since 2002.
But in a Radio Five Live interview he said the system is too complex, especially for the self-employed.
He said a flat rate of tax would be easier and bring in more revenue.
"Full of sympathy"
Mr Hart-Davis described VAT as "absurdly complicated".
HMRC's website was out of action for several hours on Thursday, the last day taxpayers could submit their return.
Thousands of taxpayers were facing an automatic £100 fine plus interest on any tax due if they missed the original midnight deadline.
HMRC now says no one who files their return before midnight on Friday will be penalised.
In light of the problems Adam Hart-Davis advocated extending the deadline into the weekend, saying he was "entirely full of sympathy" for those struggling with the site.
Despite the difficulties, HMRC said 175,000 people had managed to file their tax return online by 6pm on Thursday, 25,000 more than used the service on 31 January last year.
The Revenue said service levels were returning to normal by Thursday evening, but the BBC has continued to receive reports from taxpayers who remain unable to log on.
At the root of the problem, he described the decision by the government to amalgamate Customs & Excise and the Inland Revenue into a single department as a "great mistake".
He said the departments, merged since 2005, were too huge to "slam together".
However he remains supportive of the tax system in principle.
"We need the roads, we need the schools, we need the hospitals: we have to pay tax.
"I wish we didn't have to pay so much but we need it," he said.
Hart-Davis expects he will soon be replaced with a "new and more beautiful face" but said he would be sorry to leave because his experience fronting the HMRC campaigns had been "huge fun".