A senior government official has sparked anger by advising internet users to give fake details to websites to protect their security.
Andy Smith, an internet security chief at the Cabinet Office, said people should only give accurate details to trusted sites such as government ones.
He said names and addresses posted on social networking sites "can be used against you" by criminals.
His advice was described by Labour MP Helen Goodman as "totally outrageous".
Ms Goodman, shadow culture minister, told BBC News: "This is the kind of behaviour that, in the end, promotes crime.
"It is exactly what we don't want. We want more security online. It's anonymity which facilitates cyber-bullying, the abuse of children.
"I was genuinely shocked that a public official could say such a thing."
Mrs Goodman, MP for Bishop Auckland, in the North-East of England, said she had been contacted by constituents who have been the victims of cyber-bullying on major social networking sites by people hiding behind fake names.
Mr Smith, who is in charge of security for what he described as the "largest public services network in Europe", which will eventually be accessed by millions of people in the UK, said giving fake details to social networking sites was "a very sensible thing to do".
"When you put information on the internet do not use your real name, your real date of birth," he told a Parliament and the Internet Conference in Portcullis House, Westminster.
"When you are putting information on social networking sites don't put real combinations of information, because it can be used against you."
But he stressed that internet users should always give accurate information when they were filling in government forms on the internet, such as tax returns.
"When you are interacting with government, or professional organisations - people who you know are going to protect your information - then obviously you are going to use the right stuff.
But he said that fraudsters gather a lot of personal information "from Google, social networking sites, from email footers, all sorts of places".
He added that they were "bringing this information together and cross-correlating information and then they are using it against you".
Mr Smith's comments were backed by Lord Erroll, chairman of the Digital Policy Alliance, a not-for-profit policy studies group which claims to speak for industry and charities, who was chairing the panel. He said he had always given his date of birth as "1 April 1900".
The crossbench peer later told BBC Radio 4's PM programme Mr Smith had given people "a very good bit of advice" - particularly as banks used date of birth as a means of verifying identity.
He said cyber-bullying was "a different issue". There were "technological ways" of discovering the true identity of bullies and, he added, they could also "use your details to pretend to be you".
Asked by BBC News to clarify his remarks, Mr Smith, who is head of security at the Public Sector Technical Services Authority, said there was a "balancing act" to be struck between giving details to reputable sites and posting them on websites where the need to confirm identity was not so vital.
He said: "Don't put all your information on websites you don't trust.
"If it's somewhere you trust - and obviously with government you really do need to put accurate information. Large commercial sites you are going to put the right information.
"If you are not sure about something then just be very, very cautious of what you put up, what you expose if you really don't want to be used against you."
Culture minister Ed Vaizey said he had not seen Mr Smith's remarks but told the BBC that he "wouldn't encourage people to put false identities on the internet".
"The way of viewing this issue is that we should work with Facebook to ensure people feel secure using those sites and that there is not a threat of identity theft," he said.
"It's also important for the government to work with consumers, to educate consumers about the threat of identity theft and what kind of details we should and shouldn't put online."
Citing an anecdote about novelist Salman Rushdie - who won a battle last year to use his commonly used middle name rather than his actual first name Ahmed on his profile page - he said: "Facebook doesn't allow you to put on false details and they will take you off if they discover you have."
Simon Milner, Facebook's head of policy in the UK and Ireland, who was at the conference, also took issue with Mr Smith's comment.
He told the audience of industry experts and MPs he had a "vigorous chat" with the Cabinet Office official afterwards to persuade him to revise his view.