Eruption: A photographer captures the volcano exploding out of the earthMet office says a dense layer will cover the UK on Friday
Civil Aviation Authority says the number of flights could be 'rationed'
National Air Traffic service warning of further chaos today
Ryanair warned after saying it could fly its planes through the ash
The holiday plans of hundreds of thousands of Britons were in the balance last night as the Icelandic volcano threatened flight chaos.
A provisional five-day forecast by the Met Office shows a dense layer of ash engulfing the country on Friday – the start of the bank holiday weekend and half term.
High concentrations would cause serious delays and cancellations for air passengers, and trigger knock-on effects for flights over one of the busiest weekends of the year.
Hundreds of holidaymakers transport plans were in ruins last night, but thousands more face trouble in the coming days as the ash heads south Some planes would be able to take off and land under the ash cloud, but the Civil Aviation Authority said the number of flights may have to be 'rationed'.
The warnings have left many anxious about whether their flights at the start of half term will be disrupted. Up to two million people are expected to fly in and out of the UK over the four days of the bank holiday weekend.
Officials insisted that Britain and the rest of Europe have learned from last year’s fiasco, caused by another Icelandic volcano, when planes were grounded for six days.
The screen says it all: The arrivals board at Edinburgh airport shows how many flights were cancelled and long delays for those that were expectedIt caused misery for tens of thousands of passengers, many of whom were stranded abroad and were forced to make their way home by road, rail and sea.
Although Transport Secretary Philip Hammond is confident that the great getaway will still happen, confusion reigned in Whitehall last night.
This was largely because of the unpredictable nature of weather and volcanic activity.
Forty winks: A passenger tries to sleep after failing to secure alternative accommodation or transport after his flight was cancelled yesterdayYesterday thousands of passengers endured disruption as Scotland became a virtual no-fly zone.
British Airways cancelled all flights to and from Glasgow, Edinburgh and Newcastle, while BMI, easyJet, Flybe and Aer Lingus all scrapped services.
Ryanair claimed the cancellations were an over-reaction but went on to halt all its flights in and out of Scotland. More than 250 flights were cancelled across Europe.
After a test flight up to 41,000ft in Scottish airspace, airline boss Michael
O’Leary described the so-called ‘red zone’ of highest density volcanic ash over Scotland as a 'non-existent, mythical and a misguided invention'.
The latest satellite image showing the ash plume from the Grimsvotn volcano, under the Vatnajokull glacier in south-east IcelandThe crisis has been sparked by the eruption of Iceland's Grimsvotn volcano, which has been spewing out plumes of ash, steam and smoke since Saturday.
Scientists say high concentrations of ash can cripple aircraft engines.
Since last year's eruption, airlines and the Civil Aviation Authority have eased the rules and now allow flights through 'low density' ash clouds.
Airlines that want to fly their planes through medium and high concentrations have to convince the CAA that it is safe.
Grounded: Two Ryanair jets and a FlyBe aircraft sit on the tarmac at Edinburgh airport yesterday
Long wait: Using their suitacase as a makeshift chair, two passengers at Glasgow airport pass the time by reading a book and checking a mobile phone 
Waiting and more waiting: Passengers at Edinburgh Airport wait to board buses to other airports after their flights were cancelledIf the UK is overshadowed by high concentration ash, airlines would be forced to cancel and delay flights, and timetables could be disrupted for days.
Decisions on whether to fly are based on forecasts from the Met Office's Volcanic Ash Advisory Service, rather than real-life measurements of concentrations.
Although the Met Office has part ownership of a research plane, it is currently being used in Ireland to study 'marine organic particles'.
A dedicated Met Office atmospheric research plane, commissioned after last year's volcanic chaos, is not due to arrive until next month.
Huge power: The eruption makes a spectacular sight over the Icelandic landscape
On the ground: A car drives towards the erupting Grimsvotn volcano which has sent thousands of tonnes of volcanic ash into the sky
Stunning: As the volcano erupts, huge dark ash and storm clouds gather across the Icelandic skylineYesterday it was in talks with the German Aerospace Centre to borrow a test plane.
Last night the Met Office said it stood by its five-day prediction showing the entire UK covered in an ash cloud of the highest density from 35,000ft to 55,000ft.
A spokesman for the Met Office said: 'These are the most recent charts available and are on our website. We stand by them.
He added: 'It is based on the assumption that the volcano will continue to erupt at the same rate. It is a guide, not definitive.'
source: dailymail