Millions of travellers are to be hit by a policy change on passport renewals.

Until now, the Passport Office has granted up to nine months of extra validity for British passports renewed ahead of the expiry date.

A traveller whose passport is due to expire on 1 June 2019 could renew on 1 September 2018 and get a new document valid until 1 June 2029.

But starting this week, the long-established practice of crediting a new passport with unexpired time from the old passport has been dropped.

The change was uncovered by the Money Saving Expert website after multiple users reported they had been “short-changed” when renewing their passports.

The founder, Martin Lewis, told BBC Radio 5 Live: “We got contacted by a number of our users saying they renewed their passports in the last couple of days, they had many months remaining, and those months were not added on their new passports.”

He said that the customer service team at the Passport Office had told researchers that the policy had changed on Monday 10 September.

“To have done this without giving anyone any notice isn’t right and isn’t fair,” said Mr Lewis.

“It’s really not the way that governments should be operating on something as important as your passport. People are going to feel somewhat duped by this.”

Date line: a passport with a full nine months of extra validity (Simon Calder)

The Home Office finally acknowledged the rule change four days after it was introduced.

A spokesperson 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.”

The move appears to be a belated effort to conform with the Schengen Border Code. These are the rules which are likely to regulate access for British travellers to most EU nations after Brexit.

The code does not recognise passports which are valid for more than 10 years.

Many countries, such as Dubai and Turkey, stipulate a minimum period of validity for passports.

As a result the most rational strategy for travellers was to renew as early as possible in the nine-month window.

Once the UK leaves the European Union, the EU may insist that British passports are valid for at least three months.

But the new rule incentivises travellers to delay renewal as late as possible, causing them to risk being turned away from flights because of insufficient validity on their passports.

The change broadly reduces the useful life of an adult passport from 10 to nine and a half years, approximating to a 5 per cent price increase.

In March, the cost of British passports rose well above the rate of inflation.

Were large numbers of travellers to decide to delay renewal as late as possible, the impact on the Passport Office could be severe.

Up to now anyone whose passport is due to expire early next summer could enjoy a long “renewal window”, knowing that they had many months in which to apply.

Now, though, there could be a surge in applications shortly before the peak holiday season, leading to delays in renewals and potentially jeopardising travel plans.

While the UK remains a member of the European Union, all new passports must be EU travel documents.

From the moment of Brexit, the passport will continue to be valid as a UK travel document, but will lose all the rights associated with European Union membership.

As a result of the change, it appears that the last British “EU” passport will expire on 8 June 2029.