Leaving the European Union does not mean Britain is inevitably doomed, Jeremy Corbyn will say.
In a speech setting out Labour's approach to Brexit, the party leader will say life outside the bloc is "what we make of it together".
Mr Corbyn is facing pressure to say Labour would commit to keeping the UK in the single market.
A letter backed by more than 80 senior figures warned the leader his plans for investment in schools, hospitals and social care would be unfundable unless the UK stays in the EU single market.
Mr Corbyn will say in the speech that the reality of Brexit is "more down to earth" than the naysayers and the fervent supporters claim.
He will say: "The European Union is not the root of all our problems and leaving it will not solve all our problems.
"Likewise, the EU is not the source of all enlightenment and leaving it does not inevitably spell doom for our country.
"There will be some who will tell you that Brexit is a disaster for this country and some who will tell you that Brexit will create a land of milk and honey.
"The truth is more down to earth and it's in our hands. Brexit is what we make of it together, the priorities and choices we make in the negotiations.
"This Conservative government is damaging our country and their priorities for Brexit risk increasing the damage.
"But I know what a Labour government could do for this country and that our priorities for the Brexit negotiations are the right ones to create a country that works for the many, not the few."
A letter backed by senior Labour figures including Chuka Umunna and Lord Kinnock warned leaving the internal market would hit funding for schools, hospitals and social care.
It said: "Labour must clearly and unambiguously set as a negotiating objective the goal of remaining part of the European economic area, in order to participate on a permanent basis in the single market."
Mr Corbyn will insist that Labour will give the NHS the resources it needs through taxes on the richest.
He will say: "This is a Government that has failed our NHS pre-Brexit and during Brexit.
"And it certainly can't be trusted with the NHS post-Brexit either.
"Labour will give the NHS the resources it needs because we will raise tax on those with the broadest shoulders to pay for it, not by making up numbers and parading them on the side of a bus.
"And we will use funds returned from Brussels after Brexit to invest in our public services and the jobs of the future, not tax cuts for the richest.
"Some want to use Brexit to turn Britain in on itself. Others want to use Brexit to put rocket boosters under our current economic system's insecurities and inequalities, turning Britain into a deregulated corporate tax haven, with low wages, limited rights, and cut-price public services in what would be a destructive race to the bottom.
"Labour stands for a completely different future."