Piles of luggage have been growing since T5 first opened
Thousands of suitcases are being sent to Milan by British Airways to try to help clear a backlog of 19,000 bags at Heathrow's new Terminal 5.
BA said it would be quicker for some bags to be sorted by a Milan courier company and then driven to home or holiday addresses in mainland Europe.
The new £4.3bn terminal has been plagued by problems and a mounting pile of luggage since opening last week.
On Wednesday, 50 domestic and European flights were expected to be cancelled.
BA said its priority was to reunite passengers with their luggage as soon as possible.
More than 400 volunteer staff have been working to deal with the backlog.
A spokesman for the airline said: "We recognise there are a lot of customers who have had their bags delayed.
"The majority will be reunited with them within the coming days. And those with bags going to Milan, we will try to reunite them with their bags as soon as possible."
He estimated 10% of the bags would be transported 600 miles by lorry to Milan. The first of the lorries were already on their way, he said.
Other luggage would be flown on BA aircraft to destinations in mainland Europe, closest to where the owners were staying.
Meanwhile, bags belonging to people in the UK are being sorted at Heathrow and other airports at Gatwick, Manchester and in Scotland.
Customers are being advised they can track baggage by calling BA or going to its website and using a code issued on their baggage reclaim form.
The BA spokesman said the automated baggage system was back in use again "to some degree", but not for "large numbers of bags".
"We are working through a range of issues with BAA (airport operator) regarding T5's baggage system," he said.
The use of the Italian courier firm was an additional measure and was not unusual, he said.
"Delayed bags must undergo enhanced levels of security screening. Much of this process must be done manually rather than using the automated baggage systems," he said.
"Although the majority of delayed bags will be sorted and reflighted at Heathrow, we can reunite larger numbers of bags with their owners more quickly.
"It is quicker for bags with European mainland addresses to go to Milan for sorting and onward transport than waiting for space to appear on flights leaving Heathrow."
BA said it was too early to say how much the luggage fiasco would cost the company.
Since opening, hundreds of flights from T5 have been cancelled. By Thursday, it aims to have 92% of flights operating.