The blanket ban on flights in and out of the UK could cost businesses more than £3m a day as workers are trapped abroad, according to an absence management firm.
The travel restrictions - which have now been extended until at least 01:00 tommorrow - have seen more than 12 times the usual number of employees phone in with a transport-related absence this morning, real-time data collected by absence management consultancy FirstCare has found.
Some 20,300 employees have been unable to get to work, claiming they are affected by the volcanic ash cloud chaos, compared to fewer than 3,000 employees on a usual Monday morning, the organisation said.
Chief executive of FirstCare Aaron Ross said at an average direct cost of £168 per day of absence, the disruption will cost UK business more than £3m a day.
Travel industry body Abta has estimated that as many as 150,000 UK workers are stranded abroad and unable to get to work, meaning the total absence cost to British industry could be far higher.
Ross said: "The high level of non-medical related absence is reminiscent of the snow chaos caused at the beginning of the year."
He added: "Our nurses have been taking calls from stranded employees across the world, many of whom have been trying to return since late last week. We are also hearing from relatives who are looking after children and dependants of those stranded and as a result are unable to go to work themselves."
Ross said small firms were particularly affected by the absence caused by the ash cloud, especially as there was no indication of when the travel ban would be lifted.
The Institute of Payroll Professionals (IPP) warned businesses to prepare now to deal with an increasing number of employee absences, with many Britons stuck overseas. Alternatively, HR staff should be considering training other departments to cope with the workloads of absent colleagues, the institute said.
Lindsay Melvin, IPP chief executive, said: "The travel chaos caused by the volcanic ash cloud does not appear to be abating and payroll and HR departments need to establish contingency plans as early as now. This serious situation will inevitably affect business productivity with many employees unable to come back to work for an indefinite time."
Lawyers have confirmed that employers are within their rights to dock the pay of staff who cannot turn up to work, but unions have stressed that would be "unfair".
British Airways, together with other airlines, has carried out test flights over the UK and found no problems, leading to concerns that the flight ban is an over-reaction by UK authorities. The airline industry is said to be losing £130m a day, causing BA to call for compensation from the European Union and UK government.
FirstCare provides services to 115,000 employees across 72 organisations. Major clients include 22 NHS trusts, fire services, British Gas, and Coca-Cola Enterprises. The average sickness absence cost per employee per day of £168 is worked out based on the average salary in the NHS and replacement staff costs, which FirstCare claims is a more conservative estimate than for workers in the private sector.