Bernie Morgan told Third Sector that the bank, which is expected to use money from unclaimed assets to provide new financial products for charities, should reflect lobbying priorities put forward by existing organisations in the social investment field.
"The bank will be in a good position to lobby because it will have a very broad view of the whole sector while it works through intermediaries," she said.
"There are plenty of other organisations in the sector that do lobbying, and do it well, but I feel that their voices might be eclipsed by the scale of the wholesaler.
"We have to come to a considered view on where it will lobby and what the role might look like.
"Unless we get going on it now, we will be caught short later on in the process."
She was speaking after the Good Deals social investment conference last week. Third Sector asked Toby Eccles, development director at Social Finance, a new company formed to research the social investment market, to comment.
He said: "The expertise that the bank builds, and the research it does, will put it in a very strong position to inform government, and other funders in the arena, about how to expand the available capital in the market place."
He said the debate should focus on accountability: "If you get the accountability wrong and it's too narrow, so that it's accountable only to financiers or government, then the bank's ability to advocate sensibly will be damaged," he said.
The bank has been named in the Government's Dormant Bank and Building Society Accounts Bill as the third-place recipient for unclaimed assets after youth services and financial inclusion. A date is yet to be set for the Bill's second Commons reading.