The board of the library had hoped the plan would remain under wraps until well after Thursday's ceremony, which is the culmination of years of delays and escalating costs.
It will be a further headache for Culture Secretary Chris Smith, who had to fight to get money out of the Treasury for this year's budget to prevent some national museums from charging.
Last week, he was threatened with the closure of the Royal Opera House, which wants more funds. Now he faces charges at the British Library for the first time in 100 years.
The board, under chairman John Ashworth, former director of the LSE, and chief executive Brian Lang, has calculated that there will still not be sufficient funds to run the library with its present pounds 85m a year grant. It has drawn up a consultation paper which recommends charging users of the 11 reading rooms.
While Mr Lang admitted the existence of the document discussing charging, he would not be drawn on how much readers would have to pay, though some insiders said a pounds 5 charge was one option being considered.
He said: "There will be a proposal for a charging regime. We cannot continue to run the library as we always have. Unless funding increases, changes will have to be made. I'm not willing to preside over a declining British Library.
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