Up until last week, when British citizens renewed their passports, time remaining on the existing document was added to the new one – up to a maximum of nine months.

But passport applicants have been told this no longer applies and any remaining months will be lost if an attempt is made to renew the document early.

The change was uncovered by the consumer affairs website Money Saving Expert after members of the public got in touch.

It comes in the same week that the government released contingency advice for travelling to the EU in the event of a no-deal Brexit.

Passports are valid for 10 years and currently cost £75.50 to renew online or £85 by post, with the fee increasing in March above the rate of inflation.

The change in rules with regard to carrying over validity could lead to applicants renewing their passports as late as possible to eke out the validity.

However, many countries do not allow visitors with less than six months on the passport.

When Britain leaves the EU, Brits travelling to the continent will need at least three months’ validity on their passport.

Martin Lewis, the founder of Money Saving Expert, said: “This change affects price, because in effect passports will now only last nine and a half years.

“But it also runs the risk of more people being sent home, ruining holidays and putting many people in a worrying situation. Also, because of the timing which effectively means this shortening will take place over the Brexit period, it’s likely we’ll see an even bigger build-up of demand before next summer, when there are already long waiting times.”

On Thursday, the government published advice for travelling to the EU on a UK passport in the event of a no-deal Brexit, in which it recommended renewing a passport if it is to be older than nine years and six months after 29 March 2019.

A Home Office spokesman said: “To meet international guidelines relating to maximum passport validity, Her Majesty’s Passport Office no longer carries over any validity from a previous passport.

“This will ensure that people travelling abroad will be compliant with border entry requirements around the world.”

Jamie Grierson, Home affairs correspondent