The National Campaign Against Insurance Premium Tax has stepped up its fight against the Chancellor hiking the tax in his March budget by lobbying every MP in the country.
The Yorkshire-based lobby group claims most Conservative MPs, including leader William Hague, have lent their support.
The move follows speculation that the rate will rise from 5% to 12.5%.
Project director Philip Allott said: "Britain has the fastest rising tax burden in Europe and the increase has been greater than that of any developed country."
He added: "Our main objective is to block further increases in a tax which our members believe is already approaching a level which could be called extortionate. We will be mounting a high-profile media campaign and lobbying ministers and others at Westminster."