Ken Livingstone is to spend up to £6m on hiring consultants to analyse and monitor the effects of his congestion charge.

Amid growing financial concern about his spending on transport and policing, the mayor has invited tenders for two highly lucrative contracts.

One, worth £3m, seeks a company to undertake research and analysis on the economic and business impact of the charge. The deal would last for three or four years.

The other seeks to examine the social effect of congestion charging and offers applicants similar terms.

The consultants are asked to consider not just the charging scheme already in place but also the mayor's plan to extend the zone into west London.

The initiative has prompted strong criticism.

The Liberal Democrat transport spokeswoman on the London assembly, Lynne Featherstone said: "At Transport for London and at City Hall the mayor already employs a team of transport modellers, researchers and economists. Serious questions must be asked as to why is it necessary for them to spend £3m of taxpayers' money to outsource work which his staff have the expertise to already carry out."

She said the contracts showed the mayor was determined to extend the charging zone. "For all intents and purposes, the westward extension of the Congestion charge is a done deal. If it is not, then why would the mayor and Transport for London tender a contract worth £3m to monitor the views of those set to live in the new zone?"

Roger Evans, the Conservative transport spokesman on the London assembly said the contracts appeared excessive.

"It seems rather extravagant to spend £3m on collecting data. You can buy a bus for £250,000 so this is 12 buses that might have come in handy. Ken does appear to take a Rolls-Royce and champagne approach to life at the public expense."

But a Transport for London spokesman said it was crucial to collect data and the contract terms were not excessive.

"Prudent management procedures ensure that a full and competitive tendering process is undertaken and sufficient funding made available to provide essential research over a number of years," he said.

"The contracts are essential to continue the monitoring of the social and economic effect of the congestion charge. Such information is provided in TfL's monitoring reports."

He said contracts relating to the westward extension will only be struck if the mayor gave the go-ahead.

The congestion charge has been regarded as the crowning achievement of Mr Livingstone's time running London but it has been controversial.

Companies including John Lewis have blamed the £5 charge for a reduction in the number of customers in the city centre. Transport for London blamed this on other factors, such as last year's problems on the Central line and the drop in the number of US tourists willing to travel since September 11.

Though big business in London mainly supports the charge, there have been calls to make it easier to pay and less onerous on commerce.

Last week the business organisation London First called for the charging period to be cut by 30 minutes and to end at 6.30pm.

Mr Livingstone has warned that charges will be increased as he seeks to raise funds to finance an infrastructure development programme.