The British diplomat who wrote article 50 has claimed that Theresa May should allow the people to vote on the final terms of Brexit, following a Tory party conference which he said had signalled a desire for a hard Brexit and caused consternation across continental Europe.

Lord Kerr of Kinlochard said it would be appropriate for a general election or referendum to be held on whether the government’s deal lived up to Boris Johnson’s optimism about negotiations. The foreign secretary said during the referendum campaign that in terms of limiting freedom of movement and retaining single market membership, he was “pro cake and pro eating it”.

Kerr, a former ambassador to the US and permanent representative to the EU, who in 2003 drafted the process by which two years of exit negotiations commences, said: “Once the government, some way down this process, has established clearly what Brexit will be like, if it turns out that Boris’s policy on cake doesn’t work, then it might not be a bad idea to ask the country in a general election, or possibly another referendum, whether this is actually what it had in mind.”

He said parliament might express the view: “‘OK, start the process now and we hope you achieve the following things; if you fail to achieve them – and remember, in the referendum, you said you would achieve them – then we will want to see you again.’ Everything changes in politics but, at the moment, I do [think that is appropriate].”

The crossbench peer, who advised Margaret Thatcher, John Major and Tony Blair in EU negotiations, also suggested that the prime minister would allow parliament to vote on a motion on triggering article 50, and win with a big majority, even if two current legal cases in the high court, seeking to force the government into such a vote, failed.

“The government seems confident it is going to win the cases; I don’t know,” Kerr said. “I suspect that even if the government does win them, it would provide for some sort of vote in parliament because it seems to be that it is risk-free, actually.

Theresa May making her keynote speech to the Conservative party annual conference in Birmingham. Photograph: Christopher Thomond/The Guardian

“It wouldn’t be on a bill if the government has won the case but it would, presumably, be on some sort of motion.”

Kerr added, however, that he believed that negotiations over the financial terms of exit would be “nasty” – and that the government had potentially made life harder for itself by upsetting politicians in EU member states with its rhetoric on immigration and its apparent intention to leave the single market and the customs union.

He said that May was “definitely signalling” hard Brexit. “That may be because she is more frightened of the Brexiters in her party than she is of the soft Remainers who are clearly in full retreat at the moment.

“That may change during the negotiations as it becomes clear that Boris’s policy doesn’t work – and things are going to be tougher. At that stage, the funders of the Tory party may start saying: ‘Hang on, are we sure about this?’ May’s vision can change over time.”

Kerr, who admitted that he had not believed that article 50 would be triggered by any state when he drafted it, added of May’s comments in Birmingham: “It has played very badly in continental Europe. There is some relief that she has given a drop-dead date for article 50 but everything else she said has caused consternation.”

Kerr, who was head of the diplomatic service until 2002, and at the heart of Britain’s team during the Maastricht treaty negotiations in 1991, added that it was also a mistake for the UK not be in attendance at the recent EU summit in Slovakia. “I would go about things a slightly different way,” he said, “I would do a bit more chatting them up. I would have gone to Bratislava for the recent meeting. I can’t understand why we didn’t. We should be there. The empty chair is a very bad policy.”

Lib Dem leader Tim Farron said: “Lord Kerr confirms what the Liberal Democrats have been saying. Voting for a departure is not the same as voting for a destination. Theresa May’s speech caused the pound to crash and the rest of Europe to harden its position. Theresa May must make clear urgently that Britain’s priority in any negotiation will be to stay in the single market.”