Hackett, a senior consultant at Schofield Lothian, is gathering a fighting fund to apply for a judicial review of the institution's general meeting in May last year, which approved the fee increase.
If that review takes place and finds that the meeting was improperly conducted, the institution may be forced to return the fees.
Hackett has written to Nick Brooke, president of the RICS, to warn him to set up a contingency fund. He added: "I offered to compromise with Brooke [last year] by asking him if he could live with a 10% increase. He said: 'No – next year's budget is set in stone'."
Hackett's "Stop the Rot" campaign is also challenging the RICS' decision not to hold an extraordinary general meeting over the fees row, despite the fact that, under RICS rules, it appeared that enough members had voted to hold one.
Donations to campaign have reached about £12,500, mostly from smaller QSs and individual members. The total cost of the legal challenge is expected to be about £25,000.
The RICS declined to comment on the £2m figure, but said the possibility of a successful judicial review was "very unlikely", because the May vote was "totally valid".
Hackett's campaign has highlighted a split within the QS profession between the top nine practices – "the forum" – and smaller firms.
The larger firms are committed to negotiating with the RICS for a greater say in its policymaking. Peter Sanders, managing partner of Gardiner & Theobald, said leaving the RICS was no more than a "distant possibility". Richard Clare, senior partner of EC Harris, said that "there have been heated meetings behind closed doors" but that he was committed to staying with the RICS.
However, Andrew Hemsley, managing director of consulting at Cyril Sweett, said: "There's a slight, sneaking regard for Hackett.
There's a general feeling of 'how they can not have an EGM?'"
Smaller QSs are taking a harder line. Dave McLymont, a sole trader, said: "I might leave if the RICS continues to fail to listen to members – for example by failing to hold an EGM."
The Chartered Institute of Building has indicated that it may change its entry procedure to make it faster for disillusioned QSs to join it. Chris Blythe, president of the CIOB, said: "We could process applications in days rather than weeks. They just have to pick up the phone."
This could take the form of some kind of block transfer from firms, although professional interviews would still be held.
Keith Kirkwood, chief executive of Schofield Lothian, said he would consider transferring to the CIOB unless the RICS reformed its governance procedures.