Lord Dubs, who came to the UK as a refugee from the Nazis, accused the Government of under-counting the number of places available for unaccompanied child refugees in an administrative error.
And he said that, as a result, 130 refugee children had to wait longer before coming into the country.
Ministers faced a furious backlash in February when they announced 350 minors would arrive from Europe through the Dubs scheme - well below the 3,000 campaigners had called for.
The Home Office has now announced the total number of children resettled under the programme - officially Section 67 of the Immigration Act 2016 - will rise to 480.
Lord Dubs told the Press Association: "I'm delighted for the sake of those 130 children but shocked and disgusted that the Government has made such a mess of this.
"I don't like the way they're doing it just before the election - the Government should be ashamed of themselves."
The figure is being increased after officials discovered scores of offers to provide support for youngsters from local councils were mistakenly missed off.
Lord Dubs added: "We have told them for a long time that local authorities have offered more places, they have known for ages and have only announced it now, right before the election.
"They have been told by us and by local authorities that some are willing to offer more places and they wouldn't listen."
When he announced the 350 figure in February, Immigration Minister Robert Goodwill said it was reached following consultation with local authorities.
In a new statement to the House of Commons on Wednesday, he said: "The Government has very recently become aware that, due to an administrative error as part of collating the figures, one region pledged 130 places which were not accounted for in setting the specified number.
"In order to ensure the specified number of children to be transferred is a true reflection of the responses to that consultation, I am today announcing that, in accordance with Section 67 of the Immigration Act, the Government is increasing the specified number from 350 to 480.
"As outlined in my original statement, the specified number includes over 200 children already transferred from France as part of the Calais camp clearance."
He added the number does not include children transferred to the UK under the Dublin Regulation covering family reunion cases.
The minister said the Government remains fully committed to the implementation of its commitment under Section 67, which was introduced following a campaign spearheaded by Lord Dubs.
Mr Goodwill also stressed no eligible child has been refused transfer to the UK as a result of the error.
Home Office officials have met with counterparts in Europe to plan future transfers.
Stephen Hale, chief executive of the charity Refugee Action, described the increase as a "welcome but modest step forward".
He added: "There are far more vulnerable refugee children in Europe in need of protection."