Disable Node's Show summary in full view in D6
I've written a form_alter function for node edit form that has solved my problem:

function my_form_alter(&amp;$form, &amp;$form_state, $form_id) {
    if ($form_id == 'page_node_form') {
        $form['body_field']['teaser_include']['#default_value'] = FALSE;

        $prefix = $form['#node']-&gt;teaser;
        $str = $form['body_field']['body']['#default_value'];

        if (substr($str, 0, strlen($prefix)) == $prefix) {
            $form['body_field']['body']['#default_value'] = substr($str, strlen($prefix));
        }

    }
}


Also the teaser_include as in the question is from the node edit form which is accessible from this checkbox: