Ann Widdecombe has come under fire after she suggested science could "produce an answer" to being gay.
In an interview on Sky News, the newly elected Brexit Party MEP was asked about previous comments she made concerning gay conversion therapy.
She said she had "pointed out that there was a time when it was thought impossible for men to become women".
Labour MP Luke Pollard said Ms Widdecombe was "continuing her sick anti-LGBT campaign".
'Misrepresented quite often'
During the interview on the Ridge on Sunday programme, Ms Widdecombe, 71, was asked whether people would want to share a platform with her due to her views on homosexuality.
After referencing the scientific progress in gender reassignment, she added: "The fact that we now think it is quite impossible for people to switch sexuality doesn't mean that science may not yet produce an answer at some stage."
Pushed by the presenter on whether she thought it was a real possibility, Ms Widdecombe replied: "I don't know any more than people once knew whether it was possible for men to become women."
The MEP said she had "never claimed that such science already exists" to change someone's sexuality.
But she added: "If you simply rule out the possibility of it, you are denying people who are confused about their sexuality or discontented with it, the chances that you do give to people who want to change gender."
Her comments drew criticism on Twitter, including from Tory MP Justine Greening.
Mr Pollard wrote that he was "utterly ashamed to be represented by this vile woman.
"Being gay isn't a disease to be cured. Ann Widdecombe is continuing her sick anti-LGBT campaign."
Former Tory MP Nick Boles - who now sits as an independent - accused Ms Widdecombe of "poisonous bigotry", while comedian Adi Ray said the comments were "deplorable".
Mayor of London Sadiq Khan said she was "peddling homophobic nonsense", adding: "She may have changed her party, but she hasn't changed her stripes."
Ben Hunte, BBC News LGBT correspondent
Ann Widdecombe's comments have shocked the LGBT community.
Last summer, the government launched an LGBT Action Plan which pledged to bring forward proposals to ban so-called conversion therapy.
The report stated, in efforts to become heterosexual, therapies "can range from pseudo-psychological treatments to, in extreme cases, surgical interventions and 'corrective' rape".
And Ms Widdecombe's stance on gay conversion therapy comes at a time of great celebration for LGBT people globally.
The start of June saw the beginning of LGBT Pride Month - a celebration recognised internationally since 1970.
Pride events are under way across the UK, with many cities getting on board to recognise how far we have come in terms of visibility and representation of those of different sexualities and genders.
It will be deeply disappointing to the UK's LGBT community that elected representatives, and people with large public platforms not only hold these views but actively promote them.