Tracey Baker, 44, received six payments totalling just short of £104,000 from the Welsh government between October 2013 and June 2014.
Officials believed the account belonged to contractors Total Support Services, Croydon Crown Court heard.
Tracey Baker appearing at Croydon Crown Court
Government staff transferred the money to Baker's account after receiving a false letter claiming that the business's bank details had changed.
Baker, of Farnley Road, South Norwood, south-east London, pleaded guilty to acquiring criminal property.
However, she denies writing the false letter to the Welsh Assembly.
The charge of fraud by false representation will lie on file, Judge Jeremy Gold said.
John-Paul Macnamara, defending, said his client was a wife and mother who had spent all of her life working for the London Underground.
Judge Gold said: "You've pleaded guilty to a very serious criminal offence and there's certainly no guarantee that you won't be going straight to custody."
The case was adjourned for sentencing next month.
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