European shares slipped Friday after the previous session's rally on the European Union's debt deal, as a disappointing Italian bond auction fueled investors' skepticism about the plan to tackle the region's debt crisis.

The FTSEurofirst 300 index <.FTEU3> made its fifth week of gains, boosted by Thursday's sharp moves after policymakers struck a deal that included leveraging up a rescue fund to 1 trillion euros and a 50 percent writedown for private bondholders of Greek debt.

But some investors are concerned by the lack of detail in the outline plan.

At a sale of 10-year Italian bonds Friday, yields hit a euro-era high, underlining the country's vulnerability at the heart of the debt crisis and causing the Italian FTSE MIB <.FTMIB> to drop 1.8 percent, underperforming other exchanges.

Yesterday was encouraging, although we do not know much details on the deal, Veronika Pechlaner, a fund manager on the Ashburton European equity fund, said. We were fully invested going into the meeting.

But the Italian bond auction spooked the market. It tells us we can not relax yet and Italy has to make more progress on the restructuring side.

She has invested in BNP Paribas , HSBC and Deutsche Bank and will look to change this depending on what the details emerge on the euro zone plan as well as seeing how the economic and corporate environment improves.

Octopus Investments remained cautiously positioned.

Following the progress that risk assets have made, we are braced to see more downside from here as investors begin to look for more detail on how the EFSF (rescue fund) will be leveraged to the 1 trillion euro mark, said Lothar Mentel, chief investment officer at Octopus Investments which manages $4 billion.

The FTSEurofirst 300 index .FTEU3 closed down 0.2 percent at 1,018.14 points and ended the week up 4.1 percent - its biggest weekly gain since early October.

The benchmark index is still down 9.2 percent so far this year as concerns have grown about the macro economic environment and contagion from the euro zone debt crisis.

EARNINGS MIXED

Earnings news gave a mixed picture about how companies were holding up.

Strong third-quarter sales at Renault lifted the shares up 4.5 percent to become the top performer on the French CAC <.FCHI>, while Wacker Chemie dropped 9.8 percent to become the biggest faller on the FTSEurofirst 300 index after the world's No.2 maker of polysilicon cut its outlook. (Reporting by Joanne Frearson)

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