Labour risks losing a big chunk of its support among younger people unless it adopts an anti-Brexit stance, according to a new poll.
The YouGov survey for new pressure group Our Future Our Choice found that Labour's poll rating could drop from 39% to 30% if it goes into the next election backing Brexit.
The official position of Jeremy Corbyn's party is that it accepts the result of the 2016 referendum and is seeking a "jobs-first" Brexit which will preserve the benefits of the single market and customs union.
The new poll found that 73% of Remain voters aged under 40 would back Labour if it opposed Brexit, compared to just 39% if it supports going ahead with EU withdrawal.
Many said they would switch to Liberal Democrats in this case.
Some 54% of 18 to 24-year-olds said they expected EU withdrawal to have a negative impact on them, with just 11% thinking it will be positive.
Asked how they would vote in an election where the two largest parties support Brexit, with just the Lib Dems opposing it, the poll found Tories led Labour by 42% to 30%, with Vince Cable's party on 22%. Under-40 Remain voters split 10% Conservative, 39% Labour and 45% Lib Dem.
But if presented with an election where only Tories back Brexit and the other two main national parties oppose it, the result was Conservatives 45%, Labour 39% and Lib Dems 10%, with Labour taking an overwhelming 73% of under-40 Remainers, against 9% for Tories and 15% for Lib Dems.
Lara Spirit, a co-founder of the OFOC campaign to stop Brexit democratically, said: "The results today are absolutely clear.
"The young's support for Labour jumps by well over a third upon the party opposing Brexit.
"This is because Brexit will be a disaster for young people, and we are now demanding the Labour Party oppose, not appease, such a drastic blow to our future."
Labour MP Wes Streeting, a leading supporter of the Open Britain campaign, said: "The message to Labour is clear - as we approach the point where we have to make a choice about whether to support or oppose the Government's policy of a hard Brexit with Britain outside the Customs Union and Single Market, backing the Government would be an electoral disaster."
- The YouGov/Youthonomics/OFOC poll questioned 4,923 adults between January 29 and February 1.