LONDON, July 20 - Budget-setting for the 2012 London Olympics has been thorough but there is still uncertainty over the design of the venues and the wider benefits of the Games, a British financial watchdog said on Friday.

The National Audit Office, which scrutinises public spending for parliament, urged planners to draw up a detailed baseline for controlling the Games' 9.3 billion pound ($19 billion) budget.

It urged the government to make clear its plans for the use of Olympic venues after the Games "to address one of the remaining areas of cost uncertainty".

"The Department (for Culture, Media and Sport) must still work to contain funding and achieve value for money and should make clear what will be delivered for the public's money," John Bourn, head of the National Audit Office, said in a statement.

Fears over spiralling costs have soured the euphoria that gripped the British capital when it was awarded the Olympics and a number of parliamentary committees have voiced concerns over financial monitoring and preparations for the event.

The government more than doubled the estimated cost of staging the London Olympics in March. Billions of pounds are being spent on building sports facilities and regenerating a swathe of east London.

The National Audit Office said the budget process had been thorough and the revised funding package should cover the estimated cost of the Games provided the assumptions it was based on held good.

"There are, however, remaining areas of uncertainty including design specifications which have not yet been finalised, the impact of construction price inflation and how potential suppliers will respond to invitations to bid for work," the agency said.

The opposition Conservatives' Olympics spokesman, Hugh Robertson, said it was encouraging the National Audit Office thought the revised budget was sufficient.

But he said: "It is clear that they feel that the most likely cause of further cost overruns will be lax management."

Olympics Minister Tessa Jowell said the government had put in place rigorous cost oversight procedures and said the Olympic Delivery Authority -- overseeing construction of Olympic venues -- had hit all its major milestones, including building tunnels to carry power lines to the Olympic Park.

London's preparations for the 2012 Olympics received a glowing report from the International Olympic Committee during an inspection visit last month and the Olympic Delivery Authority said on Thursday preparations were going smoothly.