Thomson and First Choice are using coaches and a cruise ship to get 5,000 of their customers home from Alicante and Malaga.
The operator, which has 40,000 customers stranded overseas by the ash cloud in total, believes delays could be significant even after the skies are reopened.
It started land and sea-based rescue efforts this morning.
Managing director Dermot Blastland said: “Our customers are our number one priority at all times and we felt it was necessary to take action now given the current uncertainty surrounding air travel.
“Our teams have been working around the clock to put this plan in place so that our customers can return home to their families without any further delay.”
Customers in Alicante and Malaga will travel via coach to northern France, stay overnight, catch a ferry to the UK and then be transported to their UK airport.
Operator staff will accompany customers wherever possible.
Thomson’s Island Cruises ship Island Escape, which is in Madeira, will bring those on board who had been scheduled to fly home back to the UK by sea.
And Thomson’s new ship Thomson Dream will carry customers from Majorca to Barcelona.
A spokesman added: “All our customers overseas are being accommodated in hotels, the majority on an all-inclusive basis with the remaining on half-board, and resort teams are working hard to keep them informed with the latest information.”
Meanwhile, Monarch arranged a flight between Palma and Madrid this morning with coaches waiting in Madrid to take passengers home via Calais.
Managing director Tim Jeans said: “We are endeavouring to do everything we can to assist our customers, and the rescue flight from Palma is evidence of this.
“With almost 8,000 Monarch passengers stranded on the Spanish islands with no alternative means of transport back to the UK, it was important that we mobilised this flight and schedule additional rescue flights to operate as quickly as we can over the coming days.”