F1 faces another major row as it is revealed that Mercedes and Pirelli undertook a 'secret' three-day (1,000 km) test at the Barcelona circuit following the Spanish Grand Prix.

Unofficially, Pirelli suggests that it was obliged to carry out the test, especially at a time when criticism of its 2013 tyres was leading to all manner of suggestions including reverting to the 2012 compounds. However, it is the involvement of Mercedes that many find particularly worrying.

The Italian manufacturer, which safety concerns as the primary reason for the test, claims that its contract allows it to request individual teams to test with/for it when necessary. However, the big question is why the secrecy and why Mercedes.

Appearing to suffer tyre issues more than any other team, the Brackley team has been particularly confident since arriving in Monaco, team boss Ross Brawn wearing a grin like a Cheshire cat for much of the weekend.

While they await the race, which is when the Mercedes duo's challenge usually fades, rivals point to Brawn's confidence, the front row lock-out and the sheer pace of the silver cars.

As the news spread through the paddock, Red Bull's Christian Horner was particularly vocal, complaining that he only found out about the test last night.

Whilst he sympathised with Pirelli's plight in attempting to deal with the degradation issue, he expressed his anger at the secrecy surrounding the test and the fact that it was a rival team that participated and consequently - one would assume - benefitted.

This one will run and run. Watch this space.