A sample union resignation letter is provided below...
(can be found on line at www.ourunion.org)

[date]

[name and address of union]

[SEND BY CERTIFIED MAIL, RETURN RECEIPT REQUESTED]

[To the appropriate union officer, usually the Secretary]
Dear Mr./Ms.:

I hereby resign as a member of [name of union]. My resignation is effective immediately. I will continue to meet my lawful obligation of paying a representation fee to the union under its "union shop" or "agency shop" agreement with [name of your employer].

Furthermore, I object to the collection and expenditure by the union of a fee for any purpose other than my pro rata share of the union's costs of collective bargaining, contract administration, and grievance adjustment, as is my right under Ellis v. BRAC, 466 U.S. 435 (1984). Pursuant to Chicago Teachers Union v. Hudson, 475 U.S. 292 (1986), I request that you provide me with my procedural rights, including: reduction of my fees to an amount that includes only lawfully chargeable costs; notice of the calculation of that amount, verified by an independent certified public accountant; and notice of the procedure that you have adopted to hold my fees in an interest-bearing escrow account and give me an opportunity to challenge your calculation and have it reviewed by an impartial decisionmaker.

[If you pay dues by payroll deduction, include this paragraph]: Accordingly, I also hereby notify you that I wish to authorize only the deduction of representation fees from my wages. If I am required to sign a new deduction authorization form to make that change, please provide me with the necessary form.

Please reply promptly to my request. Any further collection or expenditure of dues or fees from me made without the procedural safeguards required by law will violate my rights under the Railway Labor Act and/or U.S. Constitution.

Finally, please consider this objection to be permanent and continuing in nature.

Sincerely,

"signature"

[name]

[your address]

Note: Legal decisions have recognized that resigning from a union is effective at 0001 hours the day after you mail the letter...
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If you include the paragraph on the change in your payroll deduction authorization, you should also send a copy of this letter to the payroll department of your employer and the corporation's legal department.
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[Please note: This information about your legal rights is intended to provide accurate and authoritative information at the time it was written for general educational purposes only and is not intended as the providing of legal advice or services. Facts and legal principles applicable to specific situations may vary. Any person with a legal problem should consult competent counsel and should not rely on the information contained in this website for making any legal decision.]