Leading pro-EU MPs have stepped up pressure on Theresa May to secure the "best possible relationship" with Brussels after Brexit.
A cross-party alliance of former ministers issued a joint plea for the Prime Minister to resist "false choices" - including on the issue of remaining in or quitting the European single market.
The group are part of the new Open Britain movement, which has taken over from the official Remain campaign defeated in the European Union referendum.
The organisation, which has inherited the 500,000 grassroots supporters and social media assets of Britain Stronger In Europe, is not arguing for a second referendum.
It wants to make the "positive and patriotic case" for continued close co-operation with Europe and promote the economic benefits of immigration.
The group's supporters include former ministers Anna Soubry from the Conservative Party, Liberal Democrat Norman Lamb and Labour's Pat McFadden.
In a joint article in the Sunday Times, the three acknowledged the Brexit vote had been driven by concerns over immigration but argued the UK could not cut itself off from the world.
They said: "June 23 was a moment of change. The strength of feeling is clear. Free movement of people cannot continue as it has done. It has to be reformed.
"This was not an expression of prejudice but rather a desire for managed migration and concern that rapid immigration can put pressure on public services and local communities.
"Britain must be open to talent, but with more ability to act if excessive competition in labour markets hurts our economy."
They added: "Calls for reform must sit with a positive argument about the benefits that immigration brings.
"If we interpret the referendum result as a vote for a more insular and less inclusive country, or one in which the only way to advance working people's living standards is to turn our face against the world, we will have converted a defeat into a tragedy."
In a message aimed at the Brexit camp's desire to leave the European single market, the ex-ministers said: "For an open Britain to be a fairer Britain we must reject false choices, whether between being open to the world and remaining in the EU's single market; whether between a competitive economy and protections for workers and consumers; between being open to talent and having greater control over immigration; or between embracing change and valuing tradition."
Open Britain, which will be formally launched on Friday, will also involve former deputy prime minister Nick Clegg, ex-attorney general Dominic Grieve and Labour's Chuka Umunna.
It will be run by experienced political campaigners James McGrory, Mr Clegg's former spin doctor, and Labour's Joe Carberry.
The ill-fated Remain campaign's executive director Will Straw - who was given a CBE in David Cameron's resignation honours - will not be involved.