Theresa May's new "backstop" plan to keep Britain aligned with the EU customs union after Brexit would must not last more than "a month or two", a leading Tory Eurosceptic has said.

Iain Duncan Smith, the former Conservative leader, said Brexiteers would be prepared to accept the plan only if it was strictly time-limited.

Mrs May will tell European leaders next month that she will keep the whole of the UK aligned with customs tariff rates set in Brussels as a backstop if other arrangements to avoid a hard border in Ireland are not ready by the end of the transition period.

She has rejected the EU's fallback option of keeping Northern Ireland closely aligned with EU rules because it would effectively create a border in the Irish Sea.

Mr Duncan Smith said the new backstop would be swallowed by Eurosceptics as a temporary measure because it could be useful "maybe for a month or two".