Williams is on to its third power unit on just the fourth day of Formula 1 pre-season testing which has forced a shake-up to its programme.

Deputy team principal Claire Williams says the British squad has been forced on to the backfoot at Circuit de Barcelona-Catalunya for a second straight pre-season, albeit with the problems nowhere near as serious as 12 months ago when it missed the first two-and-a-half days of testing, but this year it has suffered repeat problems with its engine reliability.

Williams confirmed it is still working with power unit supplier Mercedes to understand the full problem from its stop on the final day of the opening pre-season test, while its progress was stalled again today when Nicholas Latifi suffered an oil system issue midway through the morning.

The issue restrained Williams to just 107 laps on the opening day of the second test, which was still the joint-sixth highest tally of the 10 F1 teams, but it has led to Williams venting her frustration at missed opportunities.

“We’ve had a few issues with some engine problems which has been a little frustrating,” Williams said. “We’re on our third engine now which has obviously cost us quite a lot of track time, which is unfortunate. I suppose it is what can happen in testing and I suppose at least it is not our issue that something has gone wrong with the car.

“Apart from that it has gone well, we’re covering the run programme as we can with the limited mileage that we’ve got. I would be happy if we had got some more miles under our belt I suppose.”

“The cause of the one today was an oil system issue, so it’s on the Mercedes side.

“There were some other issues last week. The one on Friday they are still investigating so you should talk to them about that, I don’t think they’ve found the problem there. I think there was an issue with the MGU-H on the Thursday night one, an oil leak on the MGU-H.

“There was a sensor issue on Friday morning that obviously was our issue, that put us off the track for probably half an hour.”

Given the vital track time lost as a result of the stoppages with F1 testing reduced to six days this year, Williams says its test programme is having to be trimmed and it won’t be able to complete all of the runs it had planned coming to the Spanish Grand Prix venue.

“I think everyone’s probably a little frustrated because we’ve obviously got - as everyone will have a very full programme to get through, and when you lose the number of kilometres that we’ve lost it is frustrating,” she said.

“It just means you can’t prepare as best as you’d like to do ahead of going to Melbourne. You have to sacrifice certain parts of your program in favour of others.

“Obviously we wanted to do the full program but it is what it is. We’ve just got to get on with it and hope that we don’t have any further issues in the next two days and we can get as many laps in with George and Nicholas on Thursday and Friday as possible to set us up in the best possible way for Melbourne.”

Adding its 107 laps to its first test total, Williams has now completed 431 laps at pre-season testing with two days to go. Mercedes leads the way with an overall total of 673 laps, which is over 11 race distances of the Spanish Grand Prix.