More than 4.5 million people in Northern Ireland and Scotland have received a 10-second siren sound on their phones , after a rare red weather alert warned of what could be the strongest storm in generations.

The Met Office said the arrival of Storm Éowyn could bring gusts of up to 100mph and “flying debris resulting in a danger to life”.

The red warnings for Friday, an upgrade of existing amber warnings, covered all of Northern Ireland and parts of central and southern Scotland, including Edinburgh, Glasgow and Ayr.

The UK government said more than 4.5 million people had received the emergency alerts on Thursday evening, the largest real-life use of the tool to date.

The system sent a message to every compatible mobile phone in affected areas and contained guidance on how to stay safe, a Cabinet Office spokesperson said.

Police in Northern Ireland had declared a major incident as Storm Éowyn approached the region.

In the Irish republic, a rare nationwide red warning for wind was in place, issued by Met Éireann.

At first minister’s questions in the Scottish parliament on Thursday, John Swinney told the public to take the red weather warning seriously, adding that Police Scotland was due to issue an unusual “do not travel” notice.

Swinney said he would be chairing a meeting of the government’s resilience room after the FMQs session. “We have to be clear that people should not travel and that Police Scotland will issue a formal do not travel notice shortly,” he said.

He went on: “Red weather warnings are very rare. Our message is simple: please follow the advice from the Met Office and the police, take this seriously and stay safe.”

Schools were to close across Scotland’s central belt on Friday, with colleges and universities also advising students and staff to stay at home.

The Scottish parliament building in Edinburgh would also be shut on Friday, with only essential staff being granted access.

All trains across Scotland would be suspended, ScotRail said.

In Northern Ireland, the first minister, Michelle O’Neill, and the deputy first minister, Emma Little-Pengelly, urged people to stay at home if possible and check in on vulnerable people before the severe winds struck.

Schools, colleges and the courts would be closed on Friday, public transport suspended and some health appointments postponed.

The public have been warned to stay away from forests, country parks and nature reserves because of the risk of falling trees, branches and debris.

Police said the strongest winds were expected in the region since the Boxing Day storm in 1998, which caused widespread disruption.

The Police Service of Northern Ireland assistant chief constable, Davy Beck, described Storm Éowyn as an “exceptional weather event” with wind speeds of up to 100mph expected.

He said police had declared a major incident and urged the public to take the dangers seriously. “There is expected to be serious disruption across our road network, public transport, schools, health services and other public services,” said Beck. “The message from police is: do not travel, remain indoors and stay safe.”

In the Republic of Ireland, public transport had been cancelled on Friday as the country prepared for a status red warning with “extreme risk to life”. Tidal surges of up to 2.5 metres were expected across “all coastlines across the island”.

Keith Leonard, Ireland’s National Emergency Coordination Group chair, asked people to remain indoors during the storm.

Forecasters said the storm could mean power cuts and dangerous driving conditions caused by fallen trees, blown-off roofs and downed power lines. Roads, bridges and railway lines were likely to be closed.

Northern Railways, which operates across northern England, advised passengers to “avoid travelling where possible” as it cancelled services before the storm. The company said it was expecting “severe disruption” and had cancelled services across the network, including trains across the Pennines, down the Cumbrian coast and between Carlisle and Newcastle.

LNER said there would be no services north of Newcastle in either direction from 11am on Friday. Avanti West Coast advised passengers not to travel north of Preston or on its north Wales route.

Éowyn is the fifth named storm of the 2024-25 season and potentially the most destructive.

Forecasters have said it could be the strongest system the UK and Ireland has seen since Storm Debbie in September 1961, which wrecked homes, cut power and killed 12 people in Ireland and six in Northern Ireland.

The Met Office said peak rush hour wind speeds of 80-90mph were expected across Northern Ireland, with up to 100mph in some exposed locations.

The record for a gust in Northern Ireland is 124mph, in Kilkeel, County Down, in January 1974.

Experts said Éowyn could also bring torrential rain and heavy snow across the UK.

There were amber wind warnings in place for north Wales, northern England and the rest of Scotland; and a yellow wind warning across south-east England.

Jess Neumann, an associate professor of hydrology at the University of Reading, said: “Storm Éowyn is not one to underestimate – stay safe, stay prepared, and plan ahead. This storm has the potential to bring serious risks, not just to travel and property, but to your safety and wellbeing.”

Those sentiments were echoed by Prof Liz Bentley, the chief executive of the Royal Meteorological Society, who said Éowyn was a “particularly potent storm” because of a “very active jet stream”.

Data from Flightradar24 showed a BA flight from Las Vegas to Heathrow hit a speed of 814mph this week because of the strong tailwind created by the jet stream.

Bentley said: “The subsonic speed record currently stands at 835mph, so that record could well be broken in the coming hours.”

The European Storm Forecast Experiment (Estofex), a forecasting initiative involving European meteorologists, said “tornado events” could not be ruled out, with areas between Bristol and London most at risk on Friday.

As the storm approached, the Royal Society for the Prevention of Accidents (RoSPA) advised people to avoid all travel unless necessary. If people have to drive they should do it cautiously, be alert for debris and stay a safe distance from other vehicles.

At home, outdoor furniture and bins should be secured and people should “remain indoors as much as possible”. It also advised people to prepare an emergency kit with essentials such as water, non-perishable food, torches, batteries and first aid.