Sebastian Coe, the chair of the London 2012 organising committee (LOCOG), has insisted that preparations for the 2012 Olympics are on track.

There has been a month of mounting speculation over the spiralling budgets for the Games, following the sudden departure of the chair of the Olympic Delivery Authority (ODA), Jack Lemley at the end of October.

Coe, writing in the Daily Telegraph, dismissed the views of "'instant experts', many of whom have been commenting on various aspects of our preparations with varying degrees of absurdity."

Coe, who travelled with Tessa Jowell, the secretary for Culture, Media and Sport and David Higgins, the chief executive of the ODA, to Barcelona this week, said: "We saw at first hand how the Games stimulated a huge regeneration programme which changed the fortunes of the city and its people.

"I was also reminded of the absolutely crucial importance of having an ambitious vision."

It is the nature of the ambitious vision which has raised concerns. The original cost of building the venues for 2012 was estimated at £2.3bn but it is universally accepted that this cost will escalate. The cost of the regeneration element of the project – £1.5bn – is thought to be also undervalued.

Reports this week have indicated that the total cost of the Games may exceed £5bn, and that the Treasury is in favour of building a 60 per cent contingency fund.