Adults and accompanied children claiming asylum will only receive refugee status for 30 months under the changes.
Imran Hussain, from the Refugee Council charity, told the BBC that reviewing the status of refugees - who were successful, not failed, asylum seekers - would be costly and distract the Home Office from speeding up the system.
He also warned that if refugees felt they "had one foot in the departure lounge" even though their asylum claim had been accepted, then this would make it more difficult for them to hold down employment and to integrate.
But the home secretary, speaking to the BBC at the British Embassy in Copenhagen, said the importance of her changes should not be underestimated.
She said: "Honestly, I really believe these reforms are existential for us as a country and we have a real problem with a lack of control in our border system and what that does to public trust and confidence in politics - but also in the ability of the state to actually just do anything at all."
Conservative Party leader Kemi Badenoch said she believed Mahmood is "trying to do some very difficult things", adding the UK needs to update its laws linked to refugees to ensure they are "in line with reality today as opposed to reality from many years ago".
Badenoch, answering questions after a speech on Monday, said there is "a lot more" Mahmood needs to do on immigration and asylum reform.
She added: "But what I'm not going to do is criticise someone who is actually taking some tough decisions against the interests of her backbenchers."
On her visit to Denmark, Mahmood inspected dormitory accommodation for new arrivals at a reception centre in a rural location, 16 miles from the Danish capital.
She expressed approval of the conditions - warm even in the sub-zero temperatures that Denmark was experiencing, but basic bordering on austere.
She emphasised that she was fully committed to moving asylum seekers out of hotels and into similar conditions in the UK.
And she acknowledged that Labour could lose the next election if it could not keep its pledge to end the use of asylum hotels.
She said: "This is a key priority. If we fail to fulfil our promise to the British people then I would expect they will vote accordingly.
"It means losing the right to be the government of your country."
Disclaimer
This article is intended for general information purposes only and does not constitute legal advice. For advice specific to your situation, please contact our team at T & M Legis for a consultation with our Legal Experts.
