The Co-operative Group, Halifax, Virgin Holidays, Lloyds Banking Group and Vauxhall have joined Ford UK in pulling their advertising from the News of the World as Prime Minister David Cameron used PMQs to back calls for an inquiry into phone hacking.

The Co-operative Group said in a statement: "(we) have taken the decision to suspend temporarily any further advertising and promotional activity with the News of the World until the outcome of the investigation is known. The Group is a consumer-owned business which adheres to strong ethical standards. These allegations have been met with revulsion by the vast majority of members who have contacted us."

The Co-op had earlier said they would not be removing their ads from the News International paper, but relented in the face of overwhelming pressure by their members.

Ford UK became the first advertiser to withdraw from the pages of the News of the World yesterday: "We are awaiting an outcome from the News of the World investigation and expect a speedy and decisive response," they said. "Pending this response we will be using alternative media within and outside News International Group instead of placing Ford advertising in the News of the World."

Npower, T-Mobile, Dixons and several other major advertisers said in statements that they were reviewing their options with the newspaper, as it emerged that the parents of Holly Wells and Jessica Chapman, whose daughters were murdered in 2002, had been also contacted by investigators regarding possible hacking of their daughters' phones.

Other companies said that they were waiting for the investigation to conclude before making a decision. Tesco said via Facebook: "We know that you have a lot of questions surrounding recent News of the World allegations...It's now a matter for the police; like everyone, we await the outcome of their investigation."

Users of Twitter and Facebook led calls for a boycott. Andy Dawson, editor of BitterWallet.com, worked with two other users, @The_Z_Factor and @TheGreatGonzo, to set up a page where users could Tweet at the News of the World's major advertisers and ask them to pull their ads with a single click. The page had been visited more than 41,000 times as of yesterday evening, Dawson said.

"It would be great to hit the News Of The World where it hurts the most – in the pocket," Dawson added. "If it goes any way towards bringing about a cultural shift in how the tabloids operate, that would be better still. It also acts as a lesson that people aren’t prepared to be passive anymore in this age of social media – if companies want to join in and engage with their customers, they’d better be prepared for us to scream at them if we think they’re doing it wrong."

Advertising journalists said that the latest allegations would be a tipping point for many companies. "Brands will realise that consumers will react. They are being proactive and making a move on it," said John Reynolds, a journalist at Marketing Magazine. "There is no precedent. It will be difficult stuff for the execs at NOTW to handle."