Team boss Christian Horner has denied claims Red Bull flouted a cost-saving agreement on the way to winning the world championships in 2010.

On Christmas Eve, former FIA president Max Mosley claimed Red Bull spent more than was allowed under the Resource Restriction Agreement (RRA); a document agreed as a compromise to a full-blown budget cap.

On Monday, Italian reports claimed Red Bull’s over-spend was EUR 60 million, amid rumours Mercedes could be set to break the next agreement with its KERS development costs.

At the same time, F1’s smallest budget team HRT quit the FOTA teams association, a spokeswoman explaining that the body is "more for the big teams than the small ones".

The Spanish news agency EFE speculated that HRT’s departure might be the start of a deeper FOTA split ahead of crucial commercial negotiations with Bernie Ecclestone.

Increasingly isolated is Red Bull, with media sources suggesting that because the team was allegedly over-budget by 60m in 2010, that amount should be deducted from its allowed spending this year.

The situation means Red Bull is currently refusing to sign a new RRA for 2011.

"We’ve worked in accordance with the RRA limits since they were introduced," Horner told BBC Sport.

He linked the suspicions about Red Bull’s spending with earlier gripes about technical features of the title-winning RB6 car.

"Red Bull has committed its budgets wisely and it’s obviously surprising that people will feel that way, but it’s inevitable, I guess, when you’re at the front and winning races," added Horner.

Virgin team CEO Graeme Lowden did not refer to Red Bull specifically, but he said that even a breach of the "spirit" of the RRA would be "extremely disappointing".

Horner confirmed that Red Bull has not yet signed a new RRA.

"The (agreement) needs to be sorted quite quickly because at the moment it is unclear what rules we are working to in 2011 in many respects, so it’s important a solution is found and I think one will be found," he said.