Can a shareholder of a publicly listed company who has agreed to a lockup agreement with the majority shareholder of the company in order to prevent the company from being taken over?

In fact, lock-up or support agreements are common arrangements used to ensure that holders of significant blocks of shares will vote their shares in support of a plan of arrangement (or tender them to a bid, as the case may be), thus helping to ensure the success of the transaction. This is not illegitimate or improper, but rather this is the result of a carefully formulated policy that has now been in practice for several years. The Bingham case addressed this issue as well, as follows: One would think that a shareholder who makes a lockup deal like that must be taken to have been acting jointly with the proponent. However, if you make that assumption and if as a consequence of it, takeover proponents are not allowed vote shares [sic] acquired through such agreements, then the assumption could stifle enthusiasm for takeover bids. And that might not be a good thing: lockup agreements serve a useful purpose -- they can give takeover proponents some certainty that the deals they propose have a chance of success. Absent the comfort and assurance provided by a lockup agreement, fewer takeover bids might be launched; and since takeover bids are not necessarily bad, that could inhibit the fostering of an efficient capital market. (Bingham v. Ashton Mining of Canada Inc., [2007] B.C.J. 410 (B.C.S.C.) at paras. 51 and 52.)