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Toto Wolff has warned Mercedes must be prepared for the challenge from its rivals to increase from this weekend's Spanish Grand Prix.

Wolff is fully anticipating the true pecking order in F1 to materialise over the next couple of weekends following the four-race flyaway start to the season.

With virtually every team heading to Barcelona with upgrades on their cars, Wolff recognises Mercedes cannot be complacent.

"We're going to see some big upgrades from the different teams and power unit suppliers over the next few races, so now is the time we can truly assess the pecking order," said Wolff.

"Barcelona will be an interesting weekend from that perspective, so we must be prepared.

"On the one side, we are leading the constructors' championship by a good margin and our drivers hold the top two positions in the drivers' standings, so realistically speaking, it's tough to ask for more.

"However, it has not been plain sailing and we have had problems that we are working hard to solve.

"We have rivals breathing down our necks who are relentless in their chase.

"The longer regulations remain stable, the harder it is to find extra performance.

"We are pushing the limits to ensure we remain competitive, but if you push the limits then at a certain stage you risk stepping over them.

"The whole team, on both the chassis and power unit side, is working flat out to extract every millisecond from our package without compromising reliability."

Of key concern for Mercedes are the MGU-H problems that have hampered Lewis Hamilton's challenge at the last two races in China and Russia.

With team-mate Nico Rosberg winning the first four races of the season, and opening up a 43-point cushion over Hamilton, the reigning champion cannot afford further issues in these early stages.

Mercedes' executive technical director Paddy Lowe said: "The highest priority is to come back with our MGU-H problem solved, having had a repeat fault over the past two race weekends.

"The team has been working day and night to understand it and we're targeting a clean weekend all round."