Peers could try to block Theresa May from triggering Brexit unless she commits to red lines for the withdrawal negotiations – including membership of the European single market – beforehand.
Dick Newby, the new leader of the Liberal Democrats in the House of Lords, said his party would seek to hold up the formal process of leaving the EU if the prime minister did not accept certain conditions.
He added these could also include ensuring a role for the Scottish Government and other devolved administrations, suggesting the SNP might “really regret” not having a voice in the upper house.
Opposition peers outnumber the Tories in the Lords, although the move would depend on the UK Government agreeing to a parliamentary vote on Article 50.
UK Brexit Secretary David Davis ruled that out last week, but Lord Newby told the Press and Journal he was not prepared to simply accept the “game is up” and shrug his shoulders.
He said: “What we are likely to be seeking to do, if we get a vote on Article 50, is attach conditions to it and say this is the basis on which we think it should be negotiated.”
He conceded the Liberal Democrats could not block it on their own, but stressed: “We know there are a lot of Tories in the Lords who feel very strongly about staying in the EU.”
Labour peer Lord Liddle, who sits on the EU select committee, said he hoped there would be a vote, adding he would be “enthusiastic” about attaching conditions to it.
He said: “There should be full as possible access to the single market.
“One of the important conditions is that we continue to participate, if we can, in EU research programmes for universities and the Erasmus scheme.
“There should be no lessening of social and environmental standards.”
Lord Newby’s intervention came as Liberal Democrat members at the party’s conference in Brighton overwhelmingly endorsed a plan to offer the British people a referendum on the final Brexit deal.
The party will also continue campaigning for the UK to remain a member of the EU.
Describing membership of the single market as a “deal-breaker”, Lord Newby added: “I’m not prepared just to say ‘the game’s up’. I don’t accept it. Nigel Farage would not have accepted it.
“There are too many people who are shrugging their shoulders and saying: ‘Let’s make the best of it’, but….the best is grim.”
