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.

File photo: CGTN

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.

Anti-Brexit campaigners began legal action in Scottish courts on Friday to order him 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".

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."