readOGR and proj4 string

The manual page of readOGR states:

p4s 	PROJ4 string defining CRS, if default NULL, the value is read from
the OGR data set

then, if a  *.prj file is present for a *.shp,
why is the proj4string of the resulting SpatialPolygonsDataframe set 
to NA? is this a general behaviour or am I doing someting wrong?

absUTMpolysHABS2 <- readOGR("C:/ALOBO/Lidia",layer="test_TNT")

where I have:

test_TNT.avl
test_TNT.dbf
test_TNT.prj
test_TNT.shp
test_TNT.shx

with test_TNT.prj:

PROJCS["ED50_/_UTM_zone_31N_(CM_3E)",GEOGCS["ED50_/_Geographic",DATUM[
"D_European_1950",SPHEROID["International_1924",6378388.0,297.0]TOWGS8
4[,-87.0,-98.0,-121.0,0.0,0.0,0.0,0.0]],PRIMEM["Greenwich",0.0],UNIT["
Decimal_Degree",0.01745329251994330]],PROJECTION["Transverse_Mercator"
],PARAMETER["Latitude_Of_Center",0.0],PARAMETER["Longitude_Of_Origin",
3.0],PARAMETER["Scale_Factor",0.9996000000],PARAMETER["False_Easting",
500000.0],PARAMETER["False_Northing",0.0],UNIT["Meter",1.0]]

I get:

class(delme)

[1] "SpatialPolygonsDataFrame"
attr(,"package")
[1] "sp"

str(delme,max.level=2)

Formal class 'SpatialPolygonsDataFrame' [package "sp"] with 5 slots
  ..@ data       :'data.frame': 1555 obs. of  16 variables:
  ..@ polygons   :List of 1555
  ..@ plotOrder  : int [1:1555] 148 143 792 740 209 335 895 619 1127 
1160 ...
  ..@ bbox       : num [1:2, 1:2]  412000 4584000  466000 4624000
  .. ..- attr(*, "dimnames")=List of 2
  ..@ proj4string:Formal class 'CRS' [package "sp"] with 1 slots

delme at proj4string

CRS arguments: NA

Agus