The CAA advised passengers to check with airports before travelling
Airports in parts of Scotland and Northern Ireland have been closed because of risks from volcanic ash, the Civil Aviation Authority has said.
Glasgow, Prestwick and Derry are likely to be closed all day, Inverness shut in the morning and Edinburgh and Belfast are expecting to close in the afternoon.
Last month, volcanic ash from an Icelandic volcano grounded flights for six days.
BBC News website readers have been sending their own experiences of the airspace closure.
GRAEME HIGGS, BANDS ORGANISER IN NORTHERN IRELAND
The ash situation makes work like mine very risky and turns into a big loss. It has a big effect on the live music industry. I organise tours for bands and I have lots of Irish concerts this week.
The band Terrafolk, which is playing in Belfast, managed to fly in but won't be able to fly to Wales, where they are playing another concert.
The other band, Una and the Balkan Bears, haven't been able to get to Cork for the Baltimore Fiddle Fair.
The musicians are doing everything they can; taking ferries, driving ridiculous distances and getting up at the crack of dawn.
We have already paid in advance for instruments and lighting hire, flights and hotels, so we are bound to lose thousands of pounds.
Airlines are cracking down on bringing instruments on board anyway (but they are too fragile for the hold) and now all this disruption because of the ash - it's hardly worth organising concerts any more.
WASIM RASHID, GLASGOW
Wasim's wife, Shaheen, is stranded in Pakistan
My wife and three young children have been stuck in Lahore since the last ash disaster.
They were due back on April 17 but their flight was cancelled. Their flight on Pakistan International Airlines to Glasgow was the only one that was cancelled.
The airline did not arrange a separate flight for them, instead passengers were moved to subsequent scheduled flights where there was room to accommodate them.
After an aggravating struggle they were booked to come back to Glasgow on May 5, but now the airspace is closed.
I don't know what is going to happen. My wife says that the flight has been delayed for two hours.
STEWART LYE, GETTING MARRIED IN BELFAST
Stewart and Michelle are getting married on Friday
I am due to get married to Michelle in Belfast on Friday, but it looks like it is going to be a very quiet wedding.
Most of our guests are coming from England and one is coming from Abu Dhabi.
We both managed to get on the overnight ferry from Liverpool and will be there by Thursday. The best men are trekking it up to Stranraer to get the ferry, but one of the bridesmaids might not make it at all. She will have to leave work early to get the ferry.
What a way to ruin our big day. We are very sad and somewhat emotionally drained.
DAVID AND DIANE GOODWIN, GOING TO BELFAST
My wife and I are from Australia and are emigrating to Belfast. We've just arrived in London, after a two-week delay coming here because of the first ash cloud.
We now have to pick up our cat Maebh, who came on another flight eight hours later because of quarantine regulations.
We are booked on a flight to Belfast at 1740 BST but don't know if it will be cancelled.
So we've decided to hire a car, drive 400 miles to Stranraer and get the ferry. We will be in Belfast tonight. We didn't want to wait any more and we do need to take care of our cat.
Somewhere along the journey we'll find out if our flight has been cancelled and thus if we're eligible for a full refund. If not, having some certainty after all the uncertainty over the past couple of weeks is still good.
KEIR AND LEE DOE, FLYING FROM GLASGOW
Keir and Lee are going on their honeymoon. Photo: Darren Obbard
We got married on Saturday in Perth and were going on our honeymoon to Cyprus from Glasgow Airport.
Last night we watched the news and thought that we weren't going to be going on holiday at all and would miss our honeymoon.
When we got to the airport, it was a bit surreal, as it was empty and very quiet. The people who were there didn't know what was going on.
We found out that our airline, Thomas Cook, had arranged for us to go by bus to Newcastle, and we will be flying out from there. So we're very happy that we will have our honeymoon after all.
DAVE MCKENNA, GLASGOW
I was planning to visit my granddaughter in Alicante going out on a flight from Glasgow at 0630 BST today.
The airport's website showed the flight as cancelled but the airline's own website was only showing flights from 0700 BST onwards. There was no phone number we could call either to check.
So we had to go to Glasgow Airport even though we were 99.9% certain that the flight was cancelled. The staff at Glasgow Airport were so apathetic and unhelpful and didn't even offer an apology.
Quite a number of people were in the queue trying to book alternative flights. We shouldn't even have been there.
Both my mother and father are independently experiencing problems with flights. Mum is trying to leave Belfast, dad is trying to get back. I'm at home trying to get as much info as I can for both of them and it's proving to be a nightmare. Easyjet and Flybe don't seem to have updated their websites overnight, the BBC says the airports are open until 1300, the airports say no flights are arriving or leaving, my mum's flight is the only one not listed on EasyJet's website while all others say cancelled, live departure websites say the flights are going, airline telephone lines don't open to 0800. It's the usual lack of customer service, misinformation and apathy I have come to expect in the UK. Pathetic.
Simon Wynn, Bangor, Wales
I was due to be in meetings in South East England today, but now it's just not possible. I arrived at the airport this morning as no accurate information was available on the news, radio or the internet and was told the flight was cancelled! Why can't the news broadcasters report accurate flight details?
Christopher Connolly, Ballynahinch, Northern Ireland
I am due to fly to Belfast this evening. I am at university in Manchester and want to go home for an important family event, but I will not be able to make it. I hate the volcano. How, in this modern-age, can ash cause such disruption?
Lisa McReynolds, Manchester