Arsenal drifted further from the top four on Monday night after Crystal Palace pulled off a convincing 3-0 victory against the north Londoners.
The crushing defeat will lead to renewed calls for Arsene Wenger to consider his future in the Gunners' hot-seat.
Andros Townsend scored in the 17th minute to put his side in a deserved lead.
The hosts' performance improved further in the second half, and though Arsenal dominated possession, it was not enough to prevent a second goal for Palace in the 63rd minute by Yohan Cabaye.
A penalty was awarded to Palace not long afterwards after Townsend was brought down in the box by Emiliano Martinez, though some thought contact was made after the winger started to fall.
Nonetheless Luka Milivojevic converted the penalty to shore up Palace six points above the drop zone.
Should Olivier Giroud have had a penalty? He thought he'd been hauled down about 30 seconds before Townsend won a rather fortuitous spot kick at the other end. Had Arsenal got it back to 2-1 at that point their heads might not have dropped so extensively in that last 20 minutes.
MAN OF THE MATCH
Wilfried Zaha (Palace): Always lively, and given a deserved ovation at the end of the match.
Crystal Palace: Hennessey 7, Ward 7, Kelly 7, Sakho 8, Schlupp 8, Milivojevic 8, Cabaye 8, Puncheon 8, Zaha 8, Townsend 8, Benteke 7. Subs: Delaney 7, Flamini 7, McArthur 7,
Arsenal: Martinez 7, Bellerin 5, Mustafi 4, Gabriel 5, Monreal 6, Elneny 6, Xhaka 5, Walcott 5, Ozil 5, Alexis 5, Welbeck 5. Subs: Ramsey 5, Oxlade-Chamberlain 5, Giroud 5.
Read the full article on eurosport.co.uk: Arsenal’s top four hopes fading after humiliating Palace defeat