Bottas said: "They do seem very strong. No matter which kind of fuel loads or engine modes they're running, in any case they are quick.

"I think we feel at this point they're going to be a bit ahead."

There are four more days of testing next week before the first race of the season in Australia on 14-16 March.

Ferrari have consistently set fast times during four days at the Circuit de Barcelona-Catalunya, while it has taken Mercedes longer to get up to speed.

On Wednesday, world champion Lewis Hamilton, Bottas' team-mate, said Ferrari were "very strong", adding: "It appears they have a better package than last year, which means it will be a bigger challenge for us."

Hamilton finally set some quick times on Thursday morning and had posted a faster lap than Ferrari's Charles Leclerc by lunchtime on the final day of the test, but he used softer tyres to do it, and the Ferrari's underlying pace appeared clear.

But Leclerc said he believed their rivals may be 'sandbagging', or hiding their true pace, at this test.

"The performance has no sense for now because it remains testing. They are probably not pushing and we are not either. We don't know how much the others are sandbagging so we'll see at the first race."

Charles Leclerc, 21, swapped Sauber for Ferrari this season

Bottas, meanwhile, gave an insight into the personal battles he had faced over the winter in coming to terms with his 2018 season, in which he failed to win a race while Hamilton took 11 victories and the world championship.

He said: "It was a disappointing season and it kind of nearly made me angry with myself that in my six years in Formula 1 I haven't been achieving my target yet.

"Obviously, I have time yet, but I'm starting to realise you definitively only have one career, and I don't want to be in that situation again, that I feel like that and I start question myself if I could have done something better.

"I just want to make sure that this year I'll do everything I can to achieve those targets and I'm basically willing to do whatever it takes to make sure I achieve those goals.

"I'm just mentally prepared for the whole year, but it all comes down to details in this sport.

"I just have to be the best that I can. I know what I can do if I perform at my best level, I'll be able to be matching and being ahead of Lewis in qualifying or race pace. I know I can do it and now it's just hard work to be up there more consistently.

"I didn't win a single race last year, and there is only one target for the whole season - the world championship, for me personally and also for us, as a team."