British visitors are being refused entry to the US because of tough new counterterrorist procedures at American airport immigration desks. This week the Association of British Travel Agents (ABTA) warned travellers not to attempt entry using the Visa Waiver Programme if they have been arrested or convicted for any offence.

Frances Tuke, from the ABTA corporate affairs department, said: "We used to see one case a year of someone being turned away at a US airport. This year, there have been 20 cases already, often because of information that has come out while they are being grilled by an immigration officer."

The problems have arisen from the interpretation of question B on the I-94W visa waiver form, which passengers usually complete while travelling. It asks: "Have you ever been arrested or convicted for an offence or crime involving moral turpitude or a violation related to a controlled substance?"

Marjorie Ames, head of the consular information unit at the US embassy in London, said: "We advise people who have been arrested or convicted to check out our website before contacting our information line about whether they should apply for a visa before travelling."

The advice applies to people who have been arrested even if the arrest did not result in a criminal conviction; those with criminal records (the rehabilitation of offenders act does not apply to US visa law); those with certain communicable diseases; and those who have previously been refused admission or been deported from the US.

Almost all applications for a visa in Britain - which costs US$100 (£63) - have to be made in person at the embassy in London or the consulate in Belfast: the Edinburgh consulate cannot process visas.

A further warning was made to holders of British or EU passports who were born in certain Middle Eastern countries. They will be subject to special registration procedures that could delay entry. They are advised not to book immediate connecting flights.

Details of the new procedures can be found on usembassy.org.uk; inquiries and appointments for visa interviews must be made on 09055 444546 - calls cost £1.30 per minute.

· EasyJet had increased its minimum check-in time because of increased security at airports. Check-in desks will close 40, rather than 30, minutes before the flight.

· The Foreign Office has advised against all leisure travel to Syria because of the growing tension in the Middle East over Iraq.