The Briton, who clocked 39 laps, went into the lunch break just 0.010s clear of Ferrari’s Sebastian Vettel, though the German set his personal best on soft tyres, as opposed to the supersoft rubber that Button utilised.
Fresh from his controversial clash with team mate Lewis Hamilton on Sunday, Nico Rosberg was third quickest for Mercedes, having spent the majority of the day on medium tyres.
Alex Lynn completed the top four, having caused a stir earlier in the morning when his Williams was fitted with an unusual double-deck rear wing. It is believed the team were running this in a bid to get a handle on the 2017 aerodynamic regulations.
You can follow minute-by-minute timing updates on the official F1 App - go to 'Results and Standings' and click 'Test'.
Unofficial morning test times from Spain
| POS | DRIVER | TEAM | BEST LAP | GAP | LAPS |
| 1 | Jenson Button | McLaren | 1:23.753 | 39 | |
| 2 | Sebastian Vettel | Ferrari | 1:23.763 | 0.010 | 50 |
| 3 | Nico Rosberg | Mercedes | 1:25.099 | 1.346 | 54 |
| 4 | Alex Lynn | Williams | 1:26.071 | 2.318 | 41 |
| 5 | Daniel Ricciardo | Red Bull | 1:26.086 | 2.333 | 31 |
| 6 | Pascal Wehrlein | Manor | 1:26.464 | 2.711 | 50 |
| 7 | Esteban Ocon | Renault | 1:26.530 | 2.777 | 47 |
| 8 | Romain Grosjean | Haas | 1:26.669 | 2.916 | 35 |
| 9 | Pierre Gasly | Toro Rosso | 1:27.795 | 4.042 | 38 |
| 10 | Alfonso Celis | Force India | 1:28.570 | 4.817 | 34 |
Formula1.com will bring you full session reports at the end of each day's running at Barcelona.
