Late last year, the British government announced anyone classed as a British citizen would soon need a valid UK passport to enter the country, or have to get a $1300 certificate in their foreign passport.
But on Friday, the Home Office said it will now allow airlines to decide whether to accept an expired British passport alongside a valid foreign one.
Travel Agents' Association chief executive Julie White said leaving it to the airlines' discretion was risky.
"You can't rely on that and look, it's expensive, it's stressful and you've taken annual leave so our suggestion is, you really should be travelling with the right documentation.
"We're inundated with people contacting our travel agents around clarity because it really is confusing."
She said airlines could only deal with the information they'd been provided and would face fines if they got it wrong.
"So, if the person standing in front of them has got a New Zealand passport with an ETA [Electronic Travel Authorisation], they will go through a set of questions ... the liability then sits on the person travelling, which may actually be denied entry into the UK and turned around."
White said the British Embassy had not been forthcoming about how airlines would know whether a person required a new passport/ETA or not, but expected people to be questioned upon entry into the UK about whether they had a British parent.
She said the motivation behind the changes was driven by the UK's desire to tighten its borders and also its move toward digital.
"As they move along to [become] more digitally enabled, I think they'd have greater clarity on who has what rights."
She said a grace period to comply with the rules would be helpful, but with the changes coming into effect in only three days' time, thought it was unlikely.
White said some people had chosen to cancel or defer their travel due to the stress, noting insurance wouldn't cover the cost.
