Liberal Democrat leadership candidate Tim Farron has called on Britain to welcome about 60,000 non-EU migrants into the country as part of a joint European response to the growing refugee crisis.

Farron, who is fighting former health minister Norman Lamb for the right to succeed Nick Clegg, told the Observer that his party should show compassion by supporting an EU quota system under which refugees would be shared out between member states.

“We should support this because we are decent people. Our party should not have a mixed message about this. We should not turn people away,” he said.

The former Lib Dem president has written to David Cameron to say the UK should be proud of its record on taking in refugees, citing the admission of many thousands of Ugandan Asians who were expelled by President Idi Amin in 1972.

The policy had benefited all parties, and proved to be in the country’s economic interest. “First and foremost it is about compassion, but also there is enlightened self-interest,” Farron said.

His comments – before an Observer hustings for the two aspiring Lib Dem leaders in Bristol on Thursday – are the first by a senior UK politician in favour of a controversial quota system put forward recently by the European commission and backed by Germany and Italy.

They came as Channel tunnel services were disrupted again on Saturday after about 150 would-be migrants tried to storm the terminal in Calais in an attempt to board UK-bound freight services.

Asylum applications in the EU jumped by 44% in 2014 to 626,000, according to the EU statistics agency Eurostat.

Last month, commission president Jean-Claude Juncker proposed a compulsory quota system, which experts said would mean the UK taking in around 60,000 migrants this year.

The UK government, which has opted out of EU asylum policy, rejected the plan out of hand.

Cameron said that migration was better tackled on “why so many people want to leave their home countries and by taking measures to thwart the network of people who traffick them across the Mediterranean.

At an EU summit 10 days ago, several member states, including Hungary, Bulgaria and Lithuania, also rejected the compulsory share-out system – a move that infuriated the Italians and Germans who had backed it. Instead, they agreed on a voluntary scheme.

Farron’s intervention is part of an attempt to highlight the Lib Dems’ pro-EU and internationalist credentials.

Lamb said he had invited former party leader Paddy Ashdown and Baroness Williams to take part in a new foreign policy commission to address issues such as migration flows and climate change, that were now among the most serious facing political leaders across the world.

On migration, Lamb said: “The truth is that no one has a clear idea about how to address this challenge. We have to map out a way forward, otherwise there is a risk we as a society take a wrong turn with disastrous consequences.”

Farron and Lamb are focusing their campaigns on how to rebuild a party after it was reduced to a rump of just eight MPs at the general election.

At Thursday’s event, staged by the Observer in partnership with the Bristol Festival of Ideas, chief political commentator Andrew Rawnsley will challenge both men on how they intend to rebuild liberal Britain.

Ballot papers were distributed to Lib Dem members on 24 June and the result will be announced on 16 July.

In recent weeks the contest has threatened to turn nasty as Farron, a Christian, has been quizzed about whether his religious beliefs conflict with liberal attitudes.

Lamb suspended two members of his campaign team after they were found to have run a survey asking party members about what the aides regarded as Farron’s illiberal attitudes to abortion and LGBT rights.