Former Chancellor Rachel Reeves says Brexit has made the UK "poorer, weaker and less secure" as she argues the country should seek to rejoin the EU.
Debate around the issue has been sparked with Prime Minister Andy Burnham saying there needs to be an open and honest discussion about the relationship between the UK and EU.
Writing in the Sunday Times, Reeves said: "No-one in the Treasury ever doubted where my instincts were. I do believe Britain's future lies in the European Union."
In order to progress rejoining the EU, Reeves said supporters must "demonstrate tangible benefits" and ensure they "get the next steps right".
The Conservatives and Reform UK have suggested there is no desire to rejoin the EU.
Burnham has said that any changes to the relationship would require a mandate at the next general election, due by 2029.
Labour's 2024 election manifesto ruled out a return of free movement, single market membership or a customs union.
Burnham has said that any changes would require a mandate at a general election, which is next due by 2029.