Mr Brown made the admission when asked by Conservative MSP Dean Lockhart to confirm whether the new Scottish National Investment Bank would use Sterling over the next 10-15 years.
Appearing in front of Holyrood’s Economy committee, the Economy Secretary suggested the decision on future currency used by the bank was not one for him.
When asked if the bank would use Sterling, Mr Brown suggested it would to begin with but was unable to predict future changes.
“I think that would be the person that’s got the job of trying to establish certainty going forward to a 15 year period – nobody else can do that,” said Mr Brown, who is favourite to take over as SNP Depute Leader.
“Who knows what changes are going to happen over the course of that time? That’s the currency they’re going to work in, sure, when they’re started, but beyond that who can say what currency changes are going to make. I certainly can’t – I’m not a currency expert.”
Later Mr Lockhart tweeted: “Cab Sec for the Economy @KeithBrownSNP reveals to Economy Committee that he is `no currency expert’. In the week where the long awaited Growth Commission report is to be published this is worrying.”
On Friday the SNP will finally release its Growth Commission document, which has been written by former SNP MSP Andrew Wilson.
The Commission has been billed as an economic blueprint for an independent Scotland and is expected to address issues such as what currency would be used in the event of a Yes vote.