UKIP Home Affairs spokeswoman Jane Collins MEP has said the decision by the Conservatives not to withdraw from the ECHR in this parliament "undermines the security of our nation."
"Following the tragic events in Manchester the light has once again been shone on the ability of our national agencies to protect us from threats at home and abroad," she said.
"What I found extremely concerning was this attitude that we could not monitor everyone on the extremism radar because of financial concerns, yet we have a government which will continue to send billions of pounds abroad in foreign aid.
"I would rather that money was spent ensuring we could keep track of people who wish us harm, particularly home grown extremists where there are no border checks or foreign intelligence we can use.
"When she was Home Secretary from 2010 to 2016 Mrs May said she wanted to leave the ECHR, whose rules meant she could not extradite hate preacher Abu Qatada and delayed the extradition of Abu Hamza and four other hate preachers to the US.
"In a speech in April of last year Mrs May said that the ECHR 'can bind the hands of parliament, adds nothing to our prosperity, (and) makes us less secure by preventing the deportation of dangerous foreign nationals.'
"Yet in the Conservative Party manifesto they rule out leaving the ECHR before the next parliament, which could start as late as 2022, meaning those very risks she knows personally from being Home Secretary will still blight the security of this nation for at least another five years.
"It may well be that the reason JTAC have raised the terror threat to 'critical' is because the security services cannot trace someone with potential links to Monday night's bomber and the sophistication of the device would indicate there is a terror cell involved and information and training on bomb making was provided."
Ms Collins also criticised people who were focusing on the 'rights' of terrorists and hate preachers, saying "If all those people who were a known threat to this country were deported the security services would have more capability to search for others and watch people they are concerned about.
"Ipso facto those who spend their time fighting the government's wish to extradite and deport people are in reality aiding the terrorists. Many may not wish to but in practical terms, they are."
The Conservative Party manifesto states:
'We will not bring the European Union’s Charter of Fundamental Rights into UK law. We will not repeal or replace the Human Rights Act while the process of Brexit is underway but we will consider our human rights legal framework when the process of leaving the EU concludes. We will remain signatories to the European Convention on Human Rights for the duration of the next parliament.'