With heavy rain falling long before the day's running was due to get underway, Mercedes announced a change of plan, putting reserve driver Pascal Wehrlein in the car today and (last weekend's GP winner) Nico Rosberg tomorrow.

This impacted Force India's plans where Wehrlein was due to drive the VJM08 today, instead the car was handed over to Esteban Ocon, originally scheduled to drive on Wednesday.

Due to the rain there was virtually no running before lunch, and whilst teams asked for the test to be extended to Thursday, eventually, as the rain stopped, it was agreed to extend today's running until 20:00 (local time). Though there were no further downpours in the afternoon, a couple of light showers kept the drivers on their toes.

Wehrlein eventually emerged for his first run shortly after 15:00, completing a total of 67 laps before the extended session came to a close. Today's programme focused on data and vibration logging tests, in addition to power unit and control systems analysis.

Due to the conditions the German experienced a near-complete spectrum of tyres, running extreme wet, intermediate and supersofts.

"It was a very mixed day!" he admitted. "Unfortunately the weather meant that we couldn't do our full schedule. In fact, we didn't get any running at all this morning and only managed three runs on dry tyres in the afternoon. But there were definitely some positives at the end. I got to try the slicks, intermediates and wets, so it was good from my perspective to be able to compare the tyres and get some experience in all conditions - even if it wasn't ideal for the test programme.

"I can't really say if the car has improved since Barcelona," he continued, "as the conditions were very different and this is a very different track also. You could see even in the dry over the race weekend that a lot of drivers were struggling for grip and in the wet it's extremely slippery - unusually so, I would say. The grip is very low and it's very easy to lock up or run wide at some corners. It's a shame that I didn't get so much track time to build my experience - but it was great to be back in a Formula One car again and I'm looking forward to hopefully a smoother day tomorrow with Force India.

Ocon completed 76 laps for Force India as the team prepares for next week's British Grand Prix which will see the introduction of it's much anticipated B-spec chassis.

"It was a shame that we had so much rain this morning, but I'm happy that we were able to run late into the evening," he said, echoing a line that was to be heard along the pitlane. "The car was brilliant today, very enjoyable to drive and I noticed how much it had improved compared to the last time I drove in Barcelona.

"On days like this you learn so much because mileage in a Formula One car is so precious and I was able to test in both the wet and the dry. I didn't want the day to end because it was such a fantastic experience to work with the engineers and the whole team. Thank you to everyone for the experience once again."

"Despite the rain delays, Esteban took everything in his stride with a very mature approach and didn't put a foot wrong all day," said Tom McCullough, the team's chief race engineer. "The primary objective was to continue giving him as much experience as possible across the changing weather conditions. He therefore ran on the full wet tyre, the intermediate and the dry tyre compounds we had available. We also fitted some special instrumentation to the car to collect valuable correlation data for some development aero parts."

Max Verstappen was on duty for Toro Rosso, the youngster completing 97 laps on his way to posting the third best time of the day.

"It's a shame we lost so much of the morning because of the rain," he said, "but it was a good decision to extend the test by a couple of hours tonight to allow everyone to try and catch up with their programmes.

"Eventually, we managed to do some useful work, with quite a few laps, to gather extra data, which along with the information we got from the race weekend, will be very useful for the coming races."

"Despite the rain in the morning, this was a productive day's testing and we completed some useful work in the afternoon, said Phil Charles, the Faenza outfit's chief race engineer. "The programme began with set-up work aimed at taking a closer look at what happened over the race weekend, as well as doing some aero runs. Tyre work also featured on the job list and the day ended with some pit stop practice, which is always useful for the guys and the driver."

Tomorrow, 25 year old German, Marco Wittmann, the reigning DTM champion, takes over behind the wheel.

At Ferrari, Italian youngster Antonio Fuoco was making his F1 debut. Unfortunately, the 19-year-old had completed 71 laps when his session came to an end at T1, the F15-T going into the barriers, bringing out the only red flag of the day.

Thankfully, the youngster was uninjured whilst the car was only lightly damaged. At the time of the incident, Fuoco was running the soft compound and had set the second fastest time of 1.11.331.