Britain should take 50,000 more Syrian refugees from camps in the region, the Liberal Democrats will say in their manifesto, as well as re-opening the scheme to resettle lone child refugees in Europe.

Tim Farron, the Lib Dem leader, said Labour should match his pledge in their own manifesto and said the offer was a message about his party’s values.

“Labour should match this, this is a challenge to other parties and particularly to the government,” he said. “I think we need a strong opposition and you only get that with a clear alternative, this is our clear alternative.”

The Lib Dems estimate the cost of resettling an additional 50,000 refugees would require a hefty £4.3bn investment. Farron said his party believed it was worth the investment. “I don’t want us to be the kind of country who turns our back on those in desperate need, this is about Britain doing it’s fair share,” he said. “It’s not about taking all of the burden.”

While Farron has often urged more help for refugees after visits to refugee camps in Lesbos, Calais and Macedonia, the pledge is also a clear sign that his party is refocusing its election efforts on Labour voters, particularly metropolitan progressives.

“My job is to try to present a different set of value, we are a political party making this a major part of our campaign,” he said. “We believe the British people are better than the Conservative party are making us out to be.”

“It’s also a challenge to the British people to make it clear to their government that this is the kind of Britain they want, a decent country, which doesn’t forget those in need, while the Conservative government choose a path that makes Britain meaner, narrower and less thoughtful about our neighbours.”

The UK’s current programme, the Syrian vulnerable persons resettlement scheme, was launched in early 2014 in order to provide a route for selected Syrian refugees to come to the UK, prioritising the elderly, the disabled and victims of sexual violence and torture.

The scheme’s aim was to take 20,000 refugees from the Syrian region over five years. While more than 5,400 Syrians had been resettled by the end of last year, campaigners have said more urgent resettlement is needed.

The Lib Dems have also said they will re-open the so-called Dubs amendment scheme – named after the Labour peer Lord Alf Dubs who put down the amendment to the immigration bill – to bring lone child refugees to Britain who are in camps in Europe.

Since entering No 10 last year, Theresa May has resisted calls from campaigners to widen schemes to take more Syrian refugees, ending the transfer of children under the Dubs amendment earlier this year with the government claiming that local authorities had reached the limit of their resources. “We have always taken the view that we can help more Syrian refugees by putting aid into the region,” May has said.

The government argues that the UK has pledged more than £2.3bn to support those affected by the conflict, the largest ever response to a single humanitarian crisis in British history.

Farron said he did not believe the UK’s funding for Syrian refugee camps was a substitute for resettling refugees to allow them to rebuild their lives.

“I had met Syrian refugees in Cologne, welcomed into a civilised country which had the desire to do the right thing,” he said.

“They were setting up home, very quickly learning German, aeronautical engineers who will become loyal, tax-paying German citizens. As safe, sanitary and caring as a camp can be made to be, it’s still a flaming camp and it’s no way to raise your kids.”