More than a million Australians are about to find it much more complicated – and expensive – to head to Britain for holiday or work thanks to a looming law change.

From next Wednesday (February 25), British citizens – including those who are dual citizens of other nations – can enter the UK only on a British passport or an $1130 Certificate of Entitlement.

The rule applies even if the citizen usually lives in another country, such as Australia, and normally travels on a passport from that nation.

An estimated 250,000 Australian residents are dual British nationals, while many more are eligible for UK citizenship through their parents or grandparents.

They include Sandra, a former public servant who has lived in Australia for 50 years. She had not held a British passport since the 1980s and was told to apply for one as a first-timer – while also supplying her long birth certificate and her full marriage certificate.

“I’m 78 and all this bulls–t for one last trip home is ridiculous. I am disgusted,” Sandra told The Guardian.

She said an official at Britain’s passports office said they had been inundated with applications.

“They have not thought the process through for everyone impacted, and they have not looked at what this means for the many people who do not have a passport,” she said.

Next week’s rule shift is part of sweeping changes the British government says will streamline and modernise the country’s border. It follows the 2025 introduction of a £16 ($A32) electronic travel authorisation system.

But dual nationals cannot use the ETA or other electronic visas because the UK treats them as British for entry purposes. Nor can they travel on non‑UK passports any longer.

Airlines and other transport systems have been told to turn back passengers who don’t have the correct requirements.

“Australian-British or Australian-Irish dual citizens must enter the UK on their British or Irish passports, or through a certificate of entitlement in another valid passport. These dual citizens are unable to obtain an ETA or visa to enter the UK,” the federal government’s Smartraveller website warns.

“Dual citizens may not be able to board a flight to the UK without a valid document … The UK government has advised these requirements will be strictly enforced from February 25.”

All of which means budgeting for application fees, courier costs and processing times for a first passport or a renewal (£94.50 or about $A182 if done online).

Getting a new passport can take anything from two to six weeks, while Certificates of Entitlement have a similar time frame.

As the change nears, travel agents say they’ve been overwhelmed by inquiries.

Belle Goldie, who runs a boutique travel agency in south-west Sydney, told the ABC recently that 30 of her clients were affected in the next few weeks.

“I have an 85-year-old woman who’s taking her husband’s ashes with her daughter and son-in-law back to the UK for a nine-week holiday,” she said.

“Which I’ve had to unpick stage by stage; I’m talking car hires, transfers, accommodation.

“A once-in-a-lifetime trip for her to go back, and we’re having to change the date because there’s no way she will get all the paperwork in by March.”

Another agent, Ann-Catherine Jones in NSW, said many travellers and even those in the industry hadn’t absorbed “the nitty-gritty ramifications” of the change until recently.

“I thought I had a real handle on it until one of my clients said: ‘No, I’ve spoken to the British High Commission and even if my kids have never had a British passport, I still need to get them passports’,” she told the ABC.

Jones said it was tricky to navigate the details for descendants of British parents who were classified as dual citizens.

For those finding it all a bit hard, there is a third – albeit rather radical – option.

It is possible to apply to the British government to give up a citizenship. If accepted, those involved get a “declaration of renunciation” that can be used to show they are no longer British.

Bear in mind, however, this can affect the status of any future children.

In addition, citizenship can be given up only if someone has citizenship elsewhere, or will get another after renouncing it.

And, one last thing – the process can take up to six months.