siempre que ninguna de las partes lo hubiera denunciado = provided that neither of the parties has given notice of termination

(denunciar un contrato = give notice of the intent to terminate a contract/to unilaterally withdraw from a contract)

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Note added at 2 hrs (2015-08-27 12:37:18 GMT)
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I believe that in this context the Spanish original is simply an “automatic extension clause” indicating that unless one of the parties terminates the contract, it will be extended (“prorrogado”) automatically. In English this is often worded as giving “notice to terminate” or “notice not to renew.” Here are some examples:

On expiry of the Initial Term provided that neither party has given notice of termination in accordance with this clause, this Agreement will continue in full force and effect for a further 12 month term. Either party may terminate this Agreement at the end of the current term by giving at least 30 days notice in writing to the other party. http://webcache.googleusercontent.com/search?q=cache:3mC3sBC...

The Agreement provides for successive automatic one year renewal terms, provided that neither party gives notice of termination within sixty (60) days of the end of the then current term of the Agreement.
http://www.wikinvest.com/stock/Document_Security_Systems (DM...

Period of validity of the Agreement
Five years. The Agreement will be renewed automatically for two years provided that neither party gives notice to terminate.
http://europa.eu/rapid/press-release_IP-92-967_en.htm?locale...

Term/Renewal. The initial term of this Merchant Agreement shall be for the term of three (3) years (the “Initial Term”) commencing on the date this Merchant Agreement is executed by authorized officers of Merchant Bank and Processor. At the expiration of the Initial Term, this Merchant Agreement will automatically renew for successive one (1) year periods (each a “Renewal Term” and collectively with the Initial Term the “Term”) unless a party provides the other parties with notice of its intent not to renew this Merchant Agreement at least ninety (90) days prior to the expiration of the then current term.
http://webcache.googleusercontent.com/search?q=cache:PRvcxFc...

In other respects, despite being widely used on the web, I have always been under the impression that “denounce a contract” may actually be a literal rendering of “denunciar un contrato” (or the French “dénoncer un contrat”), so I have always been hesitant to use the expression. A search of official government websites (my way of checking legal terminology in English) shows that there is only one hit for “denounce the contract” on UK government websites (site:.gov.uk) which is a bilingual treaty with Uruguay from 1948, and only one hit on US government websites (site:.gov) which is a translation of a Russian contract. As for “impugn a contract” (as being void for fraud, undue influence, etc.), I assume that a contract would have to be impugned in court and, thus, the term “impugned” might not be appropriate in the context of an “automatic extension clause” such as this one.


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Note added at 3 days21 hrs (2015-08-31 07:04:10 GMT)
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Here are some definitions of “denuncia/denunciar” as used in the context of contracts and the law of obligations:


“El término ‘denuncia’ significa, en la teoría general de las obligaciones y contratos, el acto de comunicación o puesta en conocimiento por una parte contratante a la otra de la existencia de una causa de extinción (o modificación) de la situación contractual.”
http://blog.efl.es/actualidad-juridica/exigencia-de-denuncia...

“La facultad de denunciar el contrato, que es el derecho que tiene cualquiera de las partes para dar por terminado el contrato en forma unilateral.”
http://webcache.googleusercontent.com/search?q=cache:H_-PUgT...

“Denuncia de la relación obligatoria: Facultad de cualquiera de las partes de poner fin a la relación obligatoria mediante un acto enteramente libre y voluntario.”
source: Ricardo Villa-Real and Miguel Ángel del Arco Torres. “Diccionario jurídico elemental”. Granada: Comares, 2001.

Here are some bilingual dictionary definitions:

“Denunciar un contrato: give notice of termination of a contract.”
source: Enrique Alcaraz Varó and Brian Hughes. “Diccionario de terminus jurídicos, ingles-español; español-inglés. Barcelona: Ariel, 2005.

“Denunciar un contrato: to unilaterally terminate a contract; to repudiate a contract”.
source: Thomas L. West III. “Spanish-English Dictionary of Law and Business” Chattanooga, TN: Intermark Language Publications, 2012.