UK Prime Minister Boris Johnson will send a letter to the European Union asking for a Brexit delay if no deal has been agreed by 19 October, a Scottish court has been told.

Opposition MPs and rebels from Johnson's Conservative Party last month forced through a law requiring him to ask for an extension, because they want to avoid the UK leaving the EU without a deal.

Johnson has said he will abide by that law, which he has called "the surrender act," but has also said he would rather be "dead in a ditch" than ask for any delay, and has insisted Britain will leave the EU, "do or die", on 31 October.

Irish Prime Minister Leo Varadkar said, after meeting Danish Prime Minister Mette Frederiksen in Copenhagen: "My preference is that we come to an agreement and that we have a deal by the middle of October.

"But if the UK government were to request an extension, of course we would consider it. But I think most EU countries would really only consider it for a good reason, and that reason would have to be put forward. But certainly an extension would be better than 'no deal'."

Anti-Brexit campaigners began legal action in Scottish courts on Friday to order Johnson to comply with the law or have judges write to the EU on his behalf asking for the extension.

They fear Johnson may try and avoid the need to send the letter and the legal challenge says if Johnson fails to comply, the courts could impose penalties "including fine and imprisonment."

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In submissions to the Scottish Court of Session, the government accepted Johnson was obliged to send a letter to the EU asking for a delay and that if an extension was granted, Britain would also agree to it.

The government document, posted on Twitter by Jo Maugham, a lawyer involved in the case against Johnson, said: "In the event that neither of the conditions set out ... is satisfied he will send a letter in the form set out in the schedule by no later than 19 October 2019."

Johnson set out his revised proposals for a Brexit deal on Wednesday, including proposals to replace the Northern Ireland backstop - the measure intended to avoid the need for a visible border, with customs checks, on the island of Ireland.