Former Prime Minister Gordon Brown today makes his first intervention in the European referendum debate claiming it would “not be British” to leave.
Leave campaigners said Mr Brown's arguments "rang hollow" as he had a "disastrous record" while in office.
But Mr Brown – who many credit for helping swing Labour voters behind the unionist case in Scotland – will say the UK must stop being "reluctant Europeans".
In two articles ahead of the speech Mr Brown said the UK had had a tendency to "downplay" its role in Europe since World War Two choosing instead to "cling to" the special relationship with the US.
"Let's face it. Across half a century we have become reluctant Europeans,” he wrote.
“We didn't understand there was a new world in the making where influence mattered.
"Surely in a world that is getting smaller, being in Europe enables us to play a part in shaping something bigger than ourselves."
He said the UK must be at the forefront at efforts to reform the single market, bring peace and prosperity to the Middle East, curb illegal immigration, step up the fight against terrorism and push for greater co-ordination of energy policy to better exploit natural resources.
"I think most people would agree that it's not British or in tune with the Churchillian spirit to simply disengage when Ukraine is in turmoil. It is not British to retreat to Europe's sidelines when there is a common fight against illegal immigration and terrorism.
"It is certainly not British... to allow ourselves to be on the periphery when important defence and security decisions are made, not only in Nato but in the European Union."
In response, a Vote Leave spokesman said: "Gordon Brown was in a government that gave away part of our rebate and opened our borders across the EU. Lessons on the patriotic case for the EU will ring hollow from a prime minister with such a disastrous record in Europe."
Boris Johnson will kick start a UK-wide bus tour in Cornwall, a part of the country that has historically been eurosceptic.
Mr Johnson and other leading Leave campaigners have been seeking to highlight what they say is the paucity of David Cameron's EU renegotiation, with former Tory leader Iain Duncan Smith arguing the EU is harming the low paid widening inequality.
