SpatialPolygonDataFrame plotting using ggplot

I have a shape file for the greater London area. I use the readShapePoly function from the maptools package to load it in R as a SpatialPolygonDataFrame .

I want to the plot those polygons .. Which I have already done by using the basic of plot function in R. The output looks as shown in this image:

Now, I am trying to plot the same shape file using ggplot2 but it doesn't work for me. I am getting some weird lines in the graph as shown below:

The code I used was :

london.wards <- readShapePoly("~/TD/london_wards2013/london_wards2013.shp"
                          , proj4string=CRS(projString))
wards.count <- nrow(london.wards@data)
# assign id for each lsoa

london.wards@data$id <- 1:wards.count
wards.fort <- fortify(london.wards, region='id')
ggplot(wards.fort, aes(long, lat)) + geom_polygon(colour='black', fill='white')

where projString is the projection string describing the projection used for the input shape file.

You need to add an additional aesthetic, group. Assuming the polygon id is called ID, the synatx will look like:

ggplot(wards.fort, aes(x = long, y = lat, group = ID)) + 
   geom_polygon(colour='black', fill='white')