We're planning for a trip to New England next autumn. Will we be allowed to cross the border to Canada for a side trip of a day or two and then come back to the US? Stephanie Crummey, Plymouth

The short answer is "probably" – but any question involving the US Department of Homeland Security demands a longer explanation.

First, you need to apply for permission to board a plane to the United States. Most British holidaymakers qualify for the Electronic System for Travel Authorisation, better known as Esta. You must apply online, and pay $14 (£9) for the privilege. Usually, after your details are checked against a range of databases, permission is granted and remains valid for two years. (Beware of commercial sites charging an extra fee; the link cbp.gov/esta will take you straight to the official site.)

After clearing this first hurdle, you can then go ahead and book flights. When you land at Boston at the start of your trip, you will be fingerprinted and photographed at border control. Assuming all goes well, your passport will be stamped for a stay of up to three months.

During this time, you are free to cross for a short visit to Canada (or, for that matter, Mexico) without further formality and re-enter the US using the same Esta and admission stamp from your original arrival.

Bear in mind, though, that there is a theoretical chance that you will not be allowed back in; US Customs and Border Protection reserves the right to refuse any applicant, with no right of appeal. The Foreign Office reports "some cases of British nationals being denied re-entry into the US" after a short trip. To reduce this small risk, make sure you carry evidence of your flight booking home from Boston.

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