Nationwide's decision to opt out of Phorm's online behaviour tracking software , the UK's largest consumer broadband provider, which broke off its agreement with Phorm earlier this month.
The loss of TalkTalk came after a few days earlier.
Unlike BT, Phorm's agreement with TalkTalk had not extended to any form of trial.
Phorm pointed out that it retains a commercial agreement with BT while BT has stated that privacy was not a factor in its decision making.
The company's privacy record has been criticised by campaigners, who claim its targeting technology is too intrusive and were outraged that BT and Phorm did not notify consumers when they undertook trials.
Phorm's shares closed yesterday at 205p, down 15p or 7%. At 10am today its shares were down by almost 6% at 193p.
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