The UK government should pay reparations to Wales for stripping it of its natural resources and “depriving it of its inheritance”, the leader of Plaid Cymru has said ahead of the party’s annual conference.

Writing before the start of the two-day event on Friday, in Swansea, Adam Price said being governed from Westminster had left Wales unable to fulfil its potential and called for a Welsh reconstruction fund to “settle up” for a century of lost time.

“Through the long lens of our nation’s history we see a once resource-rich country ground down into crippling poverty,” said the Welsh nationalist leader in a comment piece seen by the Guardian.

“Today it’s not charity we seek but justice. British rule in Wales has left deep scars. No, it may not have been so bloody, but the human cost in blighted lives is to be measured in the millions.

“The sun at one time never set on the British Empire – but in the underground of the coalfield it never even dawned. Deprived of our inheritance we were left without the tools – the levers and pulleys – with which to prise ourselves out of the rut of poverty.”

The conference comes at the end of a week in which opposition parties in Westminster attempted to work together to draw up a strategy to stop Boris Johnson taking the UK out of the EU without a deal. Plaid’s Liz Saville Roberts, one of the party’s four MPs, was among the opposition leaders participating in the talks.

Wales voted to leave Europe by a margin of 52.5% to 47.5% in the 2016 referendum but Plaid has positioned itself as a party for remain voters, pledging to campaign to cancel Brexit in an election. The Brexit party won 19 out of 22 council areas in Wales in the European elections in May, with Plaid overtaking Labour to win second place.

The issue of reparations for the west’s colonial past has attracted increased attention in recent years, with both institutions and governments under pressure to atone for past wrongs. In August Glasgow University agreed to give £20m to fund a joint research centre with the University of the West Indies, to make amends for its historical links to the transatlantic slave trade.

The rural community of Capel Celyn was flooded in 1965 to create a reservoir to supply Liverpool and Wirral with water for industry. Photograph: Don Mcphee/The Guardian

The US Congress debated the question of whether reparations should be paid to the descendants of slaves for the first time in June, while the shadow chancellor, John McDonnell, said at last week’s party conference that a future Labour government would make reparations for Britain’s colonial past.

“The argument that the British Empire owes reparations to the people of its former colonies is powerfully well-made by the Indian politician Shashi Tharoor,” said Price. “But England’s first colony should be added to that long list of creditors.”

He pointed to the decision to flood the rural community of Capel Celyn in 1965 to create a reservoir to supply Liverpool and Wirral with water for industry. “We can’t rebuild the village of Capel Celyn, but we can rebuild our country. We need a new sense of hope and a National Reconstruction Fund to finance it,” he said.

“Consider this – Wales is the fifth largest exporter of electricity in the world, placed above energy-rich Norway and its $1tn sovereign wealth fund built on surpluses for its energy policy.

“And yet there are factories in Wales with full order books that cannot expand simply because they cannot afford to connect to the National Grid whose patchy footprint means in Wales its very name is something of a fiction.”

The Plaid leader concluded by quoting the Welsh rugby player Phil Bennett: “They’ve taken our coal, our water, our steel … What have they given us? Absolutely nothing.”