MPs are pushing for changes to Boris Johnson’s Brexit legislation to preserve the right of unaccompanied child refugees to be reunited with relatives in the UK after Britain leaves the European Union.

Liberal Democrats branded the prime minister “truly heartless” for stripping provisions to protect child migrants out of the legislation following the landslide Conservative victory in last month’s election.

A Lib Dem amendment to the EU Withdrawal Bill, tabled on Thursday, would force the government to guarantee family reunion rights following Brexit.

Acting party leader Ed Davey said failure to overturn the government’s “inhumane” position would put some of the most vulnerable children in the world at risk.

It comes after Labour put down a separate amendment requiring ministers to attempt to strike a deal with the EU to protect reunion rights.

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Labour is also pushing for changes to the bill to force Mr Johnson to seek a two-year extension to the end of 2022 to trade talks with the EU if he fails to secure agreement on a future partnership deal by the end of June.

The move, designed to stave off the danger of a no-deal crash-out at the end of 2020, is one of a range of amendments to be considered when the bill returns to the Commons on Tuesday, also including several from the DUP seeking to protect unhindered access to British markets for Northern Irish goods.

But the amendments stand scant chance of success, as Mr Johnson uses his 80-strong majority from last month’s election to steamroller his legislation through parliament by his 31 January deadline for Brexit.

Reunion rights for unaccompanied children were included in the EU withdrawal act passed by Theresa May following pressure from Labour peer Lord Dubs, who was himself a wartime child refugee from the Nazis. But the bill introduced by Mr Johnson following the election commits ministers only to reporting their policy on the issue to parliament.

Davey said: “The cruelty of the new Tory regime was laid bare when they decided to strip away the rights of unaccompanied child refugees to reunite with their families in their latest version of the Withdrawal Agreement Bill. “These are the actions of a truly heartless Prime Minister.

“If we don’t fix this, some of the most vulnerable children in the world will be even more at risk as a result of Boris Johnson’s Brexit policy.

“These are children who have already fled their homes, perhaps in fear of their lives. It is simply inhumane to prevent them from reuniting with loved ones here in the UK.

”Liberal Democrats know that this isn’t right – we must prioritise refugee family reunion. That’s why we’ve tabled an amendment challenging the Tory Government to reinstate the commitment to guarantee family reunion rights for unaccompanied child refugees after Brexit.”

The EU Withdrawal Bill passed its second reading in the Commons by a comfortable 124 votes before Christmas, and is scheduled to complete its passage through the lower house in three days next week.