Just my personal uneducated opinions below: Only a proper lawyer can help you, though you should do initial searches online to see wether you get an inkling of

Just my personal uneducated opinions below: Only a proper lawyer can help you, though you should do initial searches online to see wether you get an inkling of where you stand, and possibly reduce amount of time needed to spend on expensive lawyer. See if there are free legal councel near where you live, online or if anything can be covered by insurance. "I've developed it on my own time but the product directly relates to what my employer does" This is a bright huge red flag. Depending on the contract you signed, creating something in direct competition while employed can be grounds for assuming ownership. Using company resources, time, knowledge, clout, internal discussions and/or reputation, can be strong case for this company, against your position. Evidence and witnesses as well as your own words can be used against your position. If this IP was unrelated to company offerings, they would be less interested, and probably objectively strengthen your own case somewhat, depending on what they can prove and not. If you want to go very cheap, or avoid possible bill-hungry lawyers: Ask your boss wether they've cleared this with their internal lawyers and if you can get this confimed via a signed statement of company claims of ownership and exactly about what they claim ownership of and not. Always go internally through nearest boss or neutral intermediate in writing: Export the communications as well. Avoid contradicting your own claims unwittingly. If you get this letter and don't want to lawyer-up, I'm afraid your options are limited and you should do whatever necessary to avoid getting fired or sued. If they bluffed, you just called them on it, and can negotiate from there. Unfortunately, depending on the legal councel at this company and possible value of your work, your standing looks very weak, possibly damaging against you. This is why it's so vital to keep work and personal life divided. What may help in such situations is to do book-keeping of resources and hours spent separately for such side-project and document the process, while making sure Nothing from work is used.