THE third headliner for the 2011 Sonisphere Festival at Knebworth House is expected to be announced tonight (Monday).

Slipknot were the first band confirmed for the three-day rock event taking place in the grounds of the Hertfordshire stately home from Friday, July 8 to Saturday, July 10.

The masked American heavy metallers will make an exclusive UK performance at Knebworth next summer, most likely on the Sunday night.

A further batch of bands were due to be revealed last week, but only Scottish rockers Biffy Clyro – whose song Many of Horror has been covered by new X Factor winner Matt Cardle – were confirmed as the surprise Saturday night headliners.

This will be Biffy’s first headline slot in this country.

Biffy Clyro will headline Sonisphere at Knebworth on the Saturday night.

The rest of the announcement was delayed due to various technical and contractual reasons.

However, The Comet understands more groups will now be added to the bill tonight (Monday) around 8pm.

Tickets for the UK leg of Sonisphere went on sale on Friday and are available via the festival’s official website.

Iron Maiden, Alice Cooper and Rammstein headlined this summer’s second Knebworth festival, with Linkin Park and Metallica topping the bill in Sonisphere’s inaugural year.