Voters should be given a say on the British government's final Brexit deal, according to the former cabinet secretary, Lord Butler.
Butler, who was Margaret Thatcher’s principal private secretary before serving in the top civil service job from 1988 to 1998, told peers on Tuesday night that a second referendum vote was needed to establish "the up-to-date view of the British electorate" once talks with European Union member states have been concluded.
The former cabinet secretary argued that the UK government had "a duty to be sure before our departure becomes final", asking why pro-Brexit figures "who base their arguments for Brexit on the will of the people" now appeared "opposed to consulting the people on the outcome of the negotiations".
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"One has to suspect that they fear that they will get a different answer, but, if so, we ought to know," he said.
The remarks came as the House of Lords continued to debate the European Union (Notification of Withdrawal) Bill, a concession granted after the government lost its Supreme Court battle to bypass parliament before triggering Article 50 of the Lisbon Treaty, which formally kicks off two years of talks on Britain's departure.
Butler said he would give his backing to an amendment "requiring the government to consult the people again before our departure becomes final", although he acknowledged that such a tweak was unlikely to be "of much significance" because the political climate could shift drastically by the time negotiations are concluded.
"If there is no agreement, or if the terms of any agreement are unsatisfactory, and if there is evidence that public opinion may have changed, I expect that the Labour Party will not be as co-operative as it is now, rightly, over the passage of the Bill," he said.
"We know the position of the Liberal Democrats and of the Scottish Nationalists. The government may well be defeated in the House of Commons, as well as in this house, at the end of the negotiations.
"A matter of this importance is certainly an issue of confidence. If I am right that there is the prospect of that happening, by one route or another, the government or a new one will have to return to seeking the views of the British people – and so they should."
Lord Butler's call for a second referendum comes after two other former civil service heavyweights offered their take on the Brexit bill.
Lord O'Donnell, who preceded current incumbent Sir Jeremy Heywood as cabinet secretary, called on the government to ensure that parliament was given a "genuine vote" on any preliminary deal Britain strikes with the EU's 27 remaining member states, pointing out that other nations would be given their chance to ratify an agreement.
And Lord Macpherson, the former long-serving permanent secretary of the Treasury, urged his former department to champion "multilateralism over bilateralism" amid debates over the UK's post-Brexit trading future. Both ex-civil service chiefs signalled that they too would back amendments to the government's bill.