The lender also clarified that the decision was not based on race or religion.
HSBC was quoted by BBC as saying that it was applying a programme of strategic assessments to all of its businesses, after a $1.9bn fine in 2012 over poor money-laundering controls.
"As a result of these ongoing reviews, we have exited relationships with business and personal customers in over 70 countries.
"The services we provide to charities are no exception to this global review," HSBC added.
In the letter written to Finsbury Park Mosque on 22 July, the bank stated to close the account with effect from 22 September 2014.
Meanwhile, Finsbury Park Mosque trustee Khalid Oumar said: "The letters that have been sent and the letters that we received do not give any reason why the accounts were closed in the first place.
"That has led us to believe that the only reason this has happened is because of an Islamophobic campaign targeting Muslim charities in the UK."
