Most BAA airports have been closed since midday on Thursday
UK airports operator BAA has said that the ban on flights as a result of the volcanic ash cloud was costing it between £5m and £6m a day.
However, it said it did not believe that the closure of its airports would have a "material impact" on finances.
BAA operates six airports in the UK, including Heathrow and Stansted.
Separately, Irish airline Aer Lingus said that the disruption of the past five days had cost it 15m-20m euros ($20.2m-$26.9m; £13.2m-£17.6m).
The carrier said it was currently losing 4m-5m euros a day.
However, Aer Lingus added that it had "substantial cash reserves" and could "withstand a sustained closure of airspace".
On Tuesday morning, some flights took off from BAA airports at Glasgow and Edinburgh before airspace was temporarily closed again. Until then, all BAA's airports had been closed since midday on Thursday.
All UK airspace was due to opne at 2200BST on Tuesday.
In a statement, BAA - which is owned by Spanish firm Ferrovial - said it was working with National Air Traffic Services, airlines, regulatory authorities and the government to ensure its airports were ready to reopen at short notice.
And it said that it should have enough money to cover the losses caused by the shutdown.
"BAA entered this period of flight suspensions with sufficient available funds to mitigate the closure of British air space for a considerable amount of time," the company said.
"Right now, we don't think the airports' closure will have a material impact on our regulated airports' abilities to finance their activities," it added.
On Monday, British Airways said it had asked the European Union and the UK government for financial compensation for the disruption, adding that the shutdown was costing it £15m-£20m a day.
Also on Monday, Europe's biggest travel operator, TUI Travel - the owner of Thomson and First Choice - said the disruption was costing it between £5m and £6m a day.