The Home Office today claims only 14 out of a total of 58,800 people contacted via text, email and post were mistakenly asked if they had overstayed their visas, but campaigners believe the true number is far higher.
Indian-origin human rights and anti-racism campaigner Suresh Grover expressed shock after he received the message.
"I was absolutely shocked and quite horrified to receive the text. I thought it wasn't meant for me," he said.
"I came here with my parents in 1966, I was born in East Africa and have always had a British passport," said Grover, who is now consulting his lawyers on the matter.
The controversy gained further ground after Prime Minister David Cameron's office was asked to justify the campaign.
"The prime minister agrees with the principle of the texts. It is one of various means the Home Office contacts people who may not have the right to remain in the UK," the PM's official spokesperson said.
He acknowledged that the wording of the texts had changed since the campaign began. Originally, they included the phrase: "You are required to leave the UK as you no longer have the right to remain."
The messages now read: "Our records show you may not have leave to remain in the UK. Please contact us to discuss your case."
The messages were sent by private contractor Capita on behalf of the UK Border Agency (UKBA), which received 140 complaints.
"Most complaints occurred in the early part of the contract where Capita was, as part of the contract, updating sometimes out-of-date Home Office records," Capita claimed.
The Opposition Labour party described the Conservative-led government's tactic as "shambolic and incompetent".
News of the latest campaign came as it emerged that the UKBA plans to extend another controversial scheme by introducing "Go Home" vans across Britain.
The vans, which carry posters telling illegal migrants to leave the country or face arrest, could appear in more British cities following a trial in London, Immigration Minister Mark Harper said.
During July, they were driven around the London boroughs of Barking and Dagenham, Redbridge, Barnet, Brent, Ealing and Hounslow for a week, triggering widespread anger among anti-racism campaigners.
The country's Advertising Standards Agency had cleared the campaign of being offencive but ruled the statistics carried on the van were inaccurate.
The campaign had caused tensions within the Tory and Liberal Democrat coalition, with Lib Dem leader and business secretary Vince Cable branding it as "stupid and offencive".
Jeremy Browne, the home office minister who has since lost his job, had said he would not allow them to come back.