Weakened security laws allowed a suspected terrorist to get close to the venue for the Olympic Games five times in recent months, Labour claimed on Sunday.
Shadow home secretary Yvette Cooper demanded to know whether the incidents posed a threat to London 2012 safety and pointed the finger at watered-down Government anti-terror measures.
Court papers show that the 24-year-old, known as CF, was arrested and faces criminal proceedings after the authorities found he took trains through the Olympic Park despite being banned from the area.
His movements were picked up because he has to wear an electronic tag as part of restrictions imposed on him by an order under the Terrorism Prevention and Investigation Measures Act - referred to as a "Tpim".
CF's lawyers insist that he only used the route to visit a solicitor dealing with his legal challenge against the order - due to be heard by the High Court tomorrow - and had been wrongly advised that was OK.
But Labour said the fact that he was able to be in the capital at all - after being ordered under the previous anti-terror regime to stay out of the capital altogether - highlighted a serious problem.
Tpims replaced the control orders system previously used to restrict the movements and contacts of individuals thought to pose a risk to the public but who cannot be tried for reasons of national security.
Under the new regime the coalition stripped authorities of the right to forcibly relocate suspects, a power which had been opposed by the Liberal Democrats.
The Home Secretary has since proposed reserve powers that would enable enforced relocation in exceptional circumstances - but they would need parliamentary approval.
Experts, including the Government's independent reviewer of terror laws, had called for the relocation powers to be retained at least until after the Olympics.
Ms Cooper said the multiple breaches of the Tpim "raise very serious questions about the Home Secretary's judgment in downgrading counter terror powers in Olympic year against expert advice".
"Theresa May has very urgent questions to answer about how her decisions allowed this terror suspect who was previously banned from London to go repeatedly to Stratford, home of the Olympic Park," she said.
"The reports show that last year this terror suspect was seen as sufficiently dangerous for the Home Office and the courts to ban him from London.
Yvette Cooper demanded to know whether the incidents posed a threat to London 2012 safety
"Yet this year, because the Home Secretary downgraded control orders to Tpims, he was allowed back into the capital and was able to travel to the Olympic area as a result.
"Theresa May was warned repeatedly warned by us and experts including the independent reviewer of terrorism legislation that the new Tpims regime increased the risks with security in Olympic year, and would create significant extra pressures for the police and security services at an important time.
"These weaker counter terror powers are costing the taxpayer millions of pounds in extra police surveillance at a time when the police already face serious pressure from budget cuts and Olympic demands.
"Theresa May is making it harder for the police and security services to do their job at time when everyone wants to keep the Olympics safe.
"She needs now to accept she made the wrong decision and explain what she is doing urgently to put it right."
She added: "The Home Secretary and the courts had previously concluded this individual and others were so dangerous they had to be kept out of London to prevent terrorist activity, but now they are allowing them back."
High Court papers show CF's legal team have promised written evidence to prove he was seeing his solicitor on each occasion.
They blame the breach on "erroneous advice" given by the solicitor that the London Overground journeys were not in breach of the Tpim because they did not involve him stopping at Stratford International station.
CF was held overnight after being arrested for entering the Olympic no-go zone but was subsequently released on bail and is due to face prosecution for the breaches later this month.
The Sunday Telegraph said that in papers in the High Court proceedings, James Eadie QC, for the Home Office, states that the Home Secretary's assessment was that CF "would re-engage in terrorism-related activities" if released from the Tpim.
It quoted the submission as saying: "As CF has previously re-engaged in Islamist extremist activity, despite being on bail, previous disruptive action has not been enough to dissuade him from his involvement in Islamist extremism.
"His previous conduct has demonstrated a level of commitment to Islamist extremism, and CF is therefore determined to continue to adhere to his Islamist extremist agenda."
A Crown Prosecution Service spokeswoman said: "CF has been charged with five offences of breaching his Tpim."
She added that his next criminal court appearance was scheduled for July 27.