How to redirect to a different url inside the vue-router beforeRouteEnter hook?

I am building an admin page with Vue.js 2 and I want to prevent unauthenticated users from accessing the /admin route and redirect them to /login. For that I have used the In-Component Guard beforeRouteEnter in the Admin component like follows

...
beforeRouteEnter(to, from, next) {
  if(userNotLogedIn) {
    this.$router.push('/login');
  }
}

The problem here is that this is not defined in beforeRouteEnter hook. So what's the proper way to access $router and redirect to a different url in this case ?