Renault team chief Eric Boullier on Friday rejected all suggestions that he had promoted Bruno Senna for financial reasons - and then launched a scathing attack on dumped German driver Nick Heidfeld. As Heidfeld's management confirmed they are preparing to take legal action against Renault for breach of contract at a High Court hearing in London, due to be heard on September 19, Boullier went on the attack. In an exceptional verbal onslaught, he accused 34-year-old Heidfeld of lacking leadership and speed and said he had not become part of the Renault team since being drafted in as an emergency replacement for injured Pole Robert Kubica. Boullier did admit, however, that Renault had failed to produce a competitive car and had struggled through a disappointing 2011 season to date. He said: "We reviewed our performance and our level of motivation, a lot of things through the summertime, and I had to take some decision to clearly show some new direction. "There was an opportunity as well to assess Bruno as a driver, and this is why we moved on." Brazilian Senna, 27, who has yet to score a point after one hapless season in Formula One with the Hispania team in 2010, is the nephew of the legendary former champion the late Ayrton Senna and has a strong portfolio of personal sponsors. Senna was given Heidfeld's seat in the team on Wednesday in a move that sparked acrimony within the paddock at Spa-Francorchamps amid rumours that Renault are battling a shortfall in funding. Heidfeld is a veteran of 183 Grands Prix and has accumulated nearly 250 points in a respectable journeyman career. Boullier added: "Nick is a nice guy, but I think something did not work. His leadership didn't work in the team and when you are sometimes slower than (your team-mate) Vitaly (Petrov), in fact most of the time slower than Vitaly, it is difficult for him to push the team and to settle himself as the team leader." In a hard-hitting interview with Autosport he added: "I don't say the performance of the team was because of Nick. The car is not good enough, we have not developed the car well enough, we made mistakes as well. "But the loop is negative. So I had to change something in the team. "I had to change some things with the drivers as well to shake up and wake up everybody. I need to bring this motivation back to make sure that we can shake up everything, and it is not easy." He added that Renault hope Senna will complete the season with them, but nothing can be made certain until next month's High Court hearing. He also issued a warning to Heidfeld to be careful how he behaves while he is with the team in Belgium this weekend.. "I am a bit surprised to see him," said Boullier. "But obviously I guess he does not want to put himself in a breach situation. So as long as he keeps putting himself in a position to promote the team, and not telling anything else, then I am fine..."