Oct 30 (Reuters) - General Motors and the United Auto Workers (UAW) union have reached tentative contract agreement, UAW said on Monday, signaling an end to walkouts that have cost the No. 1 Detroit automaker more than $400 million a week.

The agreement comes after UAW President Shawn Fain ordered a surprise walkout Saturday at GM's Spring Hill manufacturing complex in Tennessee, which supplies engines to some of GM's most popular and profitable North American vehicles.

(Reporting by David Shepardson in Washington; additional reporting by Manas Mishra in Bengaluru Editing by Peter Henderson and Matthew Lewis)