The Brexit result of the EU referendum has set off a political earthquake beneath Westminster, with the Conservatives appointing a new leader in Theresa May, Jeremy Corbyn facing a challenge for his job heading up the Labour Party and Nigel Farage stepping down as Ukip's leader.

But the Liberal Democrats, reduced to just eight elected MPs at the 2015 general election, have enjoyed a membership surge since the historic EU vote last month (23 June 2016). Now one of the party's biggest hitters is returning to the frontline of British politics to take on the Tories over Brexit.

Former deputy prime minister Nick Clegg has been appointed the EU spokesperson of the Liberal Democrat's . Clegg, a former MEP who took the Lib Dems to power as part of the 2010 coalition government, has adopted a lower profile in Westminster since the last election.

However, he will know kick off his new role by unveiling a 'Brexit Challenge' project, which would see him use the help of experts, including academics and lawyers, to pitch what he's called "difficult questions" to May's new government.

"Theresa May says Brexit means Brexit, but no one actually knows what that means. Will we be in the single market or cut off from it, with all the implications that has for British jobs and our economy?," said Clegg.

"What does it mean for immigration? What about the Brits who live abroad and the Europeans who have made our country their home? How will we co-operate with our neighbours to tackle terrorism, cross-border crime and climate change?

"With no meaningful opposition from the Labour Party, no exit plan from the Government, Whitehall unprepared for the Brexit negotiations, and above all, Theresa May's refusal to seek a mandate from the people for what is in effect a new government, there is a real risk that she and her Brexit ministers won't be subject to the scrutiny and accountability which voters deserve.

"Whatever your views on Brexit, it is in everyone's interest to make sure what happens next is debated openly and scrutinised properly. So I want to make clear that we will work openly and collaboratively with people of all parties and none who believe that Britain must remain an open economy and a tolerant, outward-looking nation."

The comments come after Brexit Secretary David Davis indicated that the UK would not trigger Article 50 of the Lisbon Treaty, the mechanism to exit the EU, until 2017. The breakaway negations with Brussels are expected to take around two years, so the UK could not leave the economic and political bloc until 2019.

Tim Farron, the current leader of the Liberal Democrats, has promised to campaign on a platform of taking the UK back into the EU at the next general election, which could come in 2020. Likewise Owen Smith, a Labour leadership challenger, has promised to hold a second referendum on the EU to ratify any Brexit deal.