The company said some cheques had recently been stolen by fraudsters and then ‘doctored’ so they could be paid into fraudsters’ accounts.

The company says the frauds were “external” to Transact but it has updated its advice to users to urge them to stop sending cheques in the post.

It is now working with investigators to find out how the frauds took place and who perpetrated them. Transact has not disclosed how much money was defrauded but says clients should be refunded by banks who accepted the tampered cheques.

In a message to users Transact said: “Although we all rely on the UK postal system, it is not without its flaws. In addition to ongoing strike action, we are aware that some client cheques sent to Transact have been intercepted within the postal system and then doctored and paid into a fraudster’s bank account.

“Clearly, the bank whose controls have failed to prevent the fraud should cover any losses; however, it can take a long time for the money to be repaid and in any case dealing with fraud is worrying and unpleasant for clients.

“The fraudulent activity is external to us and we have involved the relevant authorities in helping us investigate, but we cannot be sure the problem is going to be resolved in the short term.“

The company has urged people sending large sums to use bank transfer or direct debit or consider using couriers to deliver cheques.

Jonathan Gunby, CEO Transact, said: "We encourage advisers and clients to place deposits by electronic means as it is faster and safer. We have functionality which allows advisers and clients to tell us that a deposit is en route, which wrapper it is destined for and how they wish to invest.

"Then, when we receive the money, we can apply it and invest it quickly; whereas, when we receive a cheque we have to take it to the bank and wait for it to clear. Over recent months, there have been delays in the Royal Mail service and we have been made aware that some cheques have gone missing. 

"No client has suffered any permanent loss but we are aware that a small number have needed to reclaim money from their bank because a fraudster has intercepted a cheque and tampered with it in a way that has got around the bank's controls. Our message is: please utilise electronic payments and notify us in advance by using the "Expected Deposit" functionality on Transact Online. And more generally, avoid sending cheques in the post for any purpose."