The 22-year-old Kubica missed the U.S. Grand Prix after his Montreal accident but has finished fourth in his two races since then in France a week ago and Britain on Sunday.

At Silverstone, Kubica also stood out for his spirited defence in the closing stages against Brazilian Felipe Massa's Ferrari.

"I am especially happy with Robert's performance in keeping Felipe behind him for 13 laps," Theissen told reporters.

"It's a very good feeling to know that both drivers can withstand pressure from behind. That's what we saw in the last race with Nick (Heidfeld) and now with Robert."

Germany's Heidfeld finished fifth in France after an entertaining battle with McLaren's double world champion Fernando Alonso.

Theissen agreed Kubica had stepped up in performance.

"He said to me last week in Magny-Cours that 'it's the first time I'm really confident about the tyre-car package.'

"Certainly Robert has changed his approach in terms of driving style," he added.

With Alonso and Ferrari's Kimi Raikkonen, Kubica was one of the leading drivers having most trouble in adapting to the Bridgestone tyres after Michelin withdrew last year.

In Britain on Sunday, all three seemed to have resolved their difficulties. Raikkonen won for the second race in a row while Alonso finished second to cut the advantage of championship-leading team mate Lewis Hamilton to 12 points.

BMW Sauber's eight points at Silverstone consolidated their third position overall, 47 points behind Ferrari and 72 adrift of McLaren but 25 clear of fourth-placed champions Renault.

Theissen saw little prospect of BMW narrowing the gap with the leaders soon.

"They are at least as far ahead as they used to be at the start of the season," he said. "The gap is almost unchanged. On fast tracks we are probably closer, on slow tracks the gap is bigger.

"It's important to me to keep the gap throughout the season because they are the two strongest teams and if we develop as fast as they do, that's the best result we can achieve. And then we can take the next step over the winter."