More than 200 civil servants have been disciplined for downloading pornographic images at work.
The sting reportedly began when someone was caught viewing porn
Computer use by 140,000 staff at the Department of Work and Pensions was monitored for eight months.
As a result of the sting, which began in December, one person has been prosecuted and 227 people disciplined, including 16 who were sacked.
However Liberal Democrat spokesman Steve Webb called for "zero tolerance" and for all 227 to be sacked.
The Sun newspaper reports that two million pages of porn were accessed, including images of child abuse.
A DWP spokesman said: "The DWP regularly screened its computers for potentially offensive and
"When monitoring proves such use on one of its computers, disciplinary action
will be taken and may well lead to criminal prosecution.
"In the majority of cases where monitoring has picked up apparent
inappropriate use, further investigation shows that staff innocently followed a
link from another site and were unaware of its content."
The check on all computers used by the department's 140,000 staff was ordered after
a civil servant was caught watching porn at work, claimed the paper.
Mr Webb, Lib Dem work and pensions spokesman, told BBC News Online all 227 people disciplined should have been sacked.
He said: "Any public servants who accesses porn sites at work should lose their jobs.
"It is assisting filth and paedophiles and is a security risk. There has to be a zero tolerance approach."
His Conservative counterpart, David Willetts, said the department had a high sickness rate and millions of pounds had been lost to fraud and error in benefit payments.
"Now, to add insult to injury, we learn that a shocking two million visits to pornographic websites have been traced to DWP computers," he said.
"Tony Blair promised welfare reform, but Labour have delivered a welfare shambles."