Pirelli now looks set to introduce its revised F1 tyre in stages, meaning that it will not be available as a race tyre for the Canadian Grand Prix as planned.

The decision to provide a different specification of tyre for the Montreal race has caused uproar amongst teams which feel they have a handle on the current rubber, whilst being welcomed by those suffering excessive wear problems. Pirelli insisted that the change was being made on safety grounds after suffering a spate of delaminations in Bahrain and Barcelona, but claimed that the change from the current specification would not be so great as to affect the competitive balance of the season.

According to sources, the revised rubber will only be made available for Friday morning practice at the Circuit Gilles Villeneuve, with each team getting two sets of the new tyre for the 90-minute session. The field will then revert to the existing compounds for the remainder of the weekend, while the latest specification is held over for full use at Silverstone later in the month.

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No explanation has been given for the change of heart, prompting speculation that it has been made in order to help diffuse the row over Mercedes' participation in a three-day, 1000km, tyre test immediately after the Spanish Grand Prix.

That matter is currently before the FIA, but there is an undercurrent of bad feeling in the paddock amid claims that the session was kept secret. Mercedes insists that it does not know what tyres it was given to test, a point confirmed by Pirelli's Paul Hembery claiming that batches were only signified by code numbers, but rival teams are finding it hard to believe that the Brackley-based squad won't derive some benefit from having run on the tyres destined for Montreal.

The latest move is only a partial stop to those fears, for limited laps in a 90-minute session - which could of course by blighted by rain - is hardly a direct match to three days of running, albeit a session that was ostensibly to evaluate tyres for 2014.

The revised tyres feature a Kevlar belt in place of the steel example currently in use, and which has been blamed for the delaminations suffered by Felipe Massa, Lewis Hamilton and Paul di Resta in recent weeks.