Travellers to the United States will have to hold modern, machine readable passports from next month.

The UK Passports Agency has announced that around 200,000 Britons still carry the old-style navy blue passports which will no longer allow entry into America.

With around four million UK nationals travelling to the country each year, the new security measures could hurt British trading with the US if these documents are not upgraded before they are next used for transatlantic travel.

Children will also no longer be able to enter America on their parents' passports when Washington's latest homeland security initiative begins.

Most British passport holders now own the modern maroon European Union version which are compatible with the new electronic reading technology being introduced.

However ministers are keen to get the message across to any of the 200,000 with the old passports who may be planning a holiday or business trip to the US.

The Passports Agency will also want to avoid a repeat of the problems in 1999 when a new computer system, the introduction of compulsory child passports and a move to new offices coincided with a surge in demand, huge queues and days of damaging headlines.

As well as the change in US regulations, from October 2 the price of a standard 10-year adult passport will cost will rise from £33 to £42.

A 48 page passport will go up from £40 to £54.50, a child's passport will increase from £19 to £25 and amendments made to an existing adult passport will increase from £22.50 to £42.

Foreign office minister Baroness Symons urged travellers to ensure they complied with the new American regulations.

"We have been issuing children with their own passports since 1998, but we estimate that up to four million under-16-year-olds are still travelling on a parent's passport. After October 1 they'll need their own passport to enter the States," she said.

"The key to having a trouble-free holiday is to know before you go. So take time to check you have the right and valid travel documents and, if necessary, apply in good time for a new passport. If you don't, your holiday could be over before it's even started."