The Charity Commission has criticised the one in five charities that failed to submit their annual accounts on time last year.
The 2009 figures from the regulator show that 20 per cent of charities did not send in their annual accounts or returns within 10 months of the end of their financial year.
Dame Suzi Leather, chair of the commission, said that the proportion of charities filing their returns on time had risen in recent years, but no organisation should miss its deadline.
"It's not acceptable for charities to send their annual accounts and returns late – they are accountable not just to us but, most importantly, to their donors, to those whom they exist to help and to the wider public," she said.
"Charities run the risk of damaging their own reputations and putting off potential donors if their financial affairs are not in order."
The commission will place adverts in the regional press reminding trustees of their obligations.
Charities with annual incomes of more than £10,000 in the most recent financial year must submit annual returns to the commission, and those with annual incomes of more than £25,000 must also submit their accounts.