I called the administration in Tønsberg to ask. They said that the city does not have a flag. They use a circular emblem [see below] that is not heraldically correct, and thus not officially approved. This emblem is not used on flags.
In a group of Norway regional and local flags I found two flags not identified. They are in pattern similar to the regional flags but include in the group of local flags. Tønsberg: horizontal red, white, blue, white, red (circa 4:1:4:1:4).
I do not know what the R-W-B-W-R pattern referred to by Jaume could be – it would look much like a variation on the national flag of Norway without the cross. Such designs are sometimes used for decorations – you can hang such banners in ways not acceptable for the national flag.
Flaggenmitteilungen 90 [fbn] report a flag for the Norwegian municipality of Tonsberg. Flag is three horizontal stripes of R, B+, R, the central one with white fimbriations.
I have commented upon this observation and attribution before. There is nothing – apart from the report in Flaggenmitteilungen – to suggest that this is the flag of Tonsberg. Rather, it looks more like decoration bunting or even a house flag of a shipping company.
Did Flaggenmitteilungen report their sources for this flag?
When source is not quoted it's supposed to be information issued directly from the director Mr. Gunther Mattern.
In my old hometown of T�nsberg, I was asked to write a piece for a local newspaper on the question of a city flag, and had the occasion to look into matter in the local library. What I have found out so far was that the city had used a flag in 1925 and a short while following this, and a hanging in its 1100 years celebration in 1971. The latter was divided vertically blue/white, with the older city seal from the 13th Century (castle on hill above boat on sea with inscription around, which is the COA of Tønsberg today) in the middle. The former was horizontally divided blue-white-blue with the proportions 1:3:1, and ending in a shallow swallowtail (quite possibly as in the signal flags for Alfa and Bravo, Tønsberg being a very maritime city). The colours were taken from the younger seal/arms of the city, from 1610 (castle with three towers). The BWB flag is not striking in its originality!
You can see the seal of T�nsberg at the official site of the municipality http://www.tonsberg.kommune.no/.
Seal depicts a stone tower and a wall on a hill and below those a ship and waves. The motives are, no doubt, inspired by Slottsfjellet with its castle and circular wall and with sea and fjord below. The inscription reads SIGILLVM.BVRGENSIVM.D'.TVNESBER (seal of the burg of Tunesberg). ..."
This seal is estimated to be from around 1250. Another seal was in use since around 1610 to early 20th century, when the use of the medieval seal was reverted to.
There is no royal warrant or whatever regarding this, but the impression given in the book (by providing a coloured image in place otherwise reserved only for accepted COAs) is that due to the documented use the seal is to be regarded as equal in status with other newer COAs of Norwegian communes.
There is no mention of flag (though that is not suprising, flags are rarely mentioned in the book).