A Suffolk-based alcohol rehabilitation charity lost nearly £10,000 after falling victim to fraudsters that sent emails impersonating its chief executive.
Lisa Grove, chief executive of Focus12, said the scam was carried out using a fake email account in her name to authorise payments to the criminals.
The charity’s IT systems were not compromised during the fraud and the incident, which occurred on Monday last week, was discovered and reported to the authorities on Wednesday.
It is not yet known whether the charity, which had an income of £744,099 in the year to the end of March 2014, will be able to recover the money.
Grove said the theft was "gut wrenching" and the amount stolen was equivalent to "six months’ hard work fundraising" or treatment for as many as 20 people.
She said that had the "very elaborate and very complicated" theft happened a year ago, before the charity managed to carry out internal reforms and expand its rehabilitation services, it could have been even more serious for its long-term prospects.
"A year ago it could have had a very different outcome as we were really struggling for a while," she said.
"If we hadn’t had the situation we did last year, not only would we have not been here now but secondly there would have been a very different outcome to losing that money."
The charity, which does not have a fundraising team, has set up a JustGiving page in a bid to raise £2,000 of the £10,000 it lost.
The JustGiving page showed the charity was almost halfway towards its initial target, having raised £810 at lunchtime on Monday.
The Charity Commission last week urged charities to review their financial controls to avoid falling foul of mandate fraud, in which people or organisations are persuaded to change direct debits or standing orders to bank accounts controlled by fraudsters.