He also defended his chief of staff Ed Llewellyn's decision to decline an offer from former Met police Assistant Commissioner John Yates to brief Mr Cameron on the police's phone-hacking investigation.

BBC: Prime Minister David Cameron's statement on phone hacking

In most cases, the offer is spelled out somewhere in the advertisement, according to Ed Dworsky of Mouseprint.org, a consumer advocacy Web site.

CNN: Watch out for the 'free' stuff

Ed Miliband told The Guardian on Saturday he wanted to offer tax breaks to persuade companies to pay more to low paid workers.

BBC: Andy Burnham calls for ban on zero hours contracts

Ed Prado, who runs Tradebonds, claims to be able to offer his retail customers prices up to 1.5 percentage points lower than they could get from traditional brokers.

ECONOMIST: E-bonds, licensed to kill

But in cases where researchers notify companies and offer to cooperate in order to solve security problems, a smarter approach to fixing those flaws has largely prevailed: Ed Felten, for instance, a Princeton researcher, faced legal threats in 2003 when he and colleagues exposed security vulnerabilities in Diebold voting machines.

FORBES: Indian Researcher Who Showed E-Voting Security Flaws Jailed, Denied Bail, Charged With Theft