post__not_in query

I’m using the following to display a list of posts from category with ID 27 but I would also like to exclude a post with ID of 297. Not too sure what I’m doing wrong.

<?php /* Start the Loop */ ?>
<?php
$featuredPosts = new WP_Query();
$featuredPosts->query('cat=27&post__not_in=297');
while ($featuredPosts->have_posts()) : $featuredPosts->the_post(); ?>
<li><a href="<?php the_permalink() ?>"><?php the_title() ?></a></li>
<?php endwhile; ?>

Any help would be much appreciated.