Labour has urged Conservative MPs to defy the government and vote to restore a commitment to family reunion for child refugees in Brexit legislation, saying there was a “moral argument” to protect desperately vulnerable young people.

The joint letter to Tory MPs was written by Alf Dubs, the Labour peer whose amendment to the EU withdrawal bill to help refugee children was accepted by Theresa May’s government, and Keir Starmer, the shadow Brexit secretary who is the current favourite to become the next Labour leader.

The commitment was dropped in the revised version of the Brexit bill tabled by the Conservatives after Boris Johnson won a majority in December. The letter comes ahead of a Commons vote on a Labour amendment seeking to restore the plan.

The letter cites the UK’s “proud record of supporting refugees”, including helping children ahead of the second world war with the Kindertransport scheme, which saw Lord Dubs, whose father was Jewish, flee Czechoslovakia in 1939 as a child.

“Boris Johnson may have won a majority in parliament, but he did not win the moral argument to absolve himself of responsibility to some of the most vulnerable people in the world,” the letter adds.

The letter, co-signed by Keir Starmer, says there is a ‘moral argument’ for the government to protect vulnerable young people. Photograph: Aaron Chown/PA

While it seems extremely unlikely that many Tory MPs will rebel to back the Labour plan, the amendment and letter form part of a wider opposition effort to signal a continued intent to try to scrutinise the Brexit process.

The withdrawal agreement bill, which sets out the legislative framework for departure, passed its second reading just before Christmas with a majority of 124, a hugely symbolic moment for the Tories after May’s Brexit plan was repeatedly defeated in the Commons.

The bill faces three days of scrutiny this week in the committee stage, where amendments can be tabled.

Among others drawn up are proposals from Labour and the Lib Dems seeking to remove a new element committing the government to ending the post-departure transition period at the end of 2020, even if negotiations on final arrangements for trade and other areas have not been completed.

Introducing the debate for the government, the Brexit secretary, Steve Barclay, said the commitment on the date showed the government was “determined to honour its promise to the British people and to get Brexit done”.

He said: “With the EU and UK committed to a deal by the end of 2020 in the political declaration, now with absolutely clarity on the timetable we’re working to, the UK and the EU will be able to get on with it.”

But a series of MPs intervened to warn that the deadline was unhelpful. The Green MP, Caroline Lucas, said: “There could be perfectly constructive negotiations going ahead, which he will be prepared to throw away if they can’t fit into the arbitrarily short time of 11 months.”

Speaking for Labour, the shadow Brexit minister, Paul Blomfield, said that while his party accepted “that the general election has changed the landscape”, this did not mean the government could act with impunity.

“I think members opposite also need to recognise that although under our electoral system the arithmetic in this place is very clear, a majority of the British people voted for parties that weren’t of the mind of the Conservative manifesto, and who wanted to give the British public a further say,” Blomfield argued.

“I say that, not to deny the reality of the voting in this place, but that the members opposite ought to have some caution about the way that they approach this issue and claim authority from the British people.”

Blomfield added: “There needs to be a voice for the approaching-55% of people in this country who were uncomfortable with the direction that the Conservative party manifesto offered. Whilst the result of the general election was clear, it doesn’t mean that the government can proceed without question, without challenge, without scrutiny.”