The Civil Aviation Authority launched its largest ever Atol refund scheme this morning, but the claims process for forward Thomas Cook bookings was hit by delays due to “unprecedented demand”.

Atol’s reimbursement programme is expected to process more than 360,000 booking refund claims for 800,000 people.

By 11:45am on Monday October 7, 11,500 claims had been submitted, and that number rose to 60,000 by the end of the day. The first direct debit payments were due to be made last night, with the others due by October 14.

The scheme was launched after the last flight in a two-week repatriation programme touched down in Manchester at 8.31am on Monday, after completing its flight from Orlando.

Head of Atol, Andy Cohen, said the CAA was fully-focused on the refund process, having completed the largest ever peacetime repatriation of British citizens.

“It’s not time for celebration,” he said. “We move on now to getting 800,000 people their money back.”

But soon after the refunds process was launched, the CAA’s Twitter feed apologised to customers who had been getting error messages.

Earlier, Richard Moriarty, the CAA’s chief executive, said: “The sheer number of refunds means this will be the largest ever Atol reimbursement programme. The CAA is working around the clock with our partners so that customers receive their refunds as quickly as possible.”

Adam French, a consumer rights expert at Which?, said: “It is understandably frustrating for some Thomas Cook customers that they are now facing problems trying to submit their refunds because of high volumes of online traffic.

“It is vital that customers persevere and do not turn to alternative websites, as we’ve seen scammers seeking to take advantage of this situation by setting up convincing fake refund websites to fleece customers.”