Which U- or V-class submarine served the longest?

Which was the last U- or V-class submarine in service?

No single boat can be identified with certainty, but Danish-operated examples appear to have lasted until about 1959.

The available service lists often give only a year rather than a month, which prevents a certain identification. Royal Navy examples generally left service earlier, while transferred boats continued under allied flags. The Danish-operated group appears to have remained until 1959 and is the strongest candidate for the latest service. A precise 'last boat' claim requires commission and decommission dates detailed enough to distinguish vessels within the same year.