Suella Braverman is facing demands from Sadiq Khan to launch an urgent review of how dozens of people once held in Manston holding centre were abandoned without food or accommodation in the capital. The mayor of London has also raised concerns with the home secretary that overcrowding and poor safeguarding in hotels housing people seeking asylum has led to reports of sexual assaults against children. Earlier it emerged that Braverman was facing a legal challenge over the conditions at Manston in Kent, which is housing thousands of people seeking refuge in the UK. On Thursday, the home secretary flew into Manston in a chinook helicopter to visit the centre for the first time. The Home Office said more than 1,000 people had been moved off-site from Manston within the past five days. At one point, 4,100 people including children and pregnant women were living in a space designed for 1,600. The Guardian on Wednesday reported that a group of asylum seekers from the Kent facility had been left at Victoria railway station with nowhere to stay and many wearing flip-flops. On Thursday, it was revealed that a teenage boy was allegedly raped by a man in his 30s at a hotel used to house refugees in east London. In a letter to Braverman, Khan wrote: “I am shocked by the shameful reports that the Home Office abandoned dozens of people seeking asylum from Manston immigration centre in central London, cold, hungry and without accommodation. “I am also deeply concerned by reports of sexual assault against children and severe overcrowding at hotels that the Home Office are using to house asylum seekers. “This situation is a clear dereliction of your department’s statutory responsibilities and safeguarding duties towards those in its care and represents a complete failure of duty. The Home Office must conduct an urgent and thorough review to ensure that the ongoing crisis is addressed and that this can never happen again. “I would also ask you to confirm whether or not people seeking asylum have been abandoned without accommodation before.” Lawyers on behalf of the charity Detention Action and a woman held at the facility in Kent sent an urgent pre-action letter to the Home Office on Tuesday. It represents the first action against the home secretary for “the unlawful treatment of people held at the facility”. The pre-action letter, sent by Duncan Lewis solicitors, said the woman, from a non-European country, “was unlawfully detained by the home secretary at the Manston facility in egregiously defective conditions”. The complaint also includes “serious threats to the safety of children”, the charity said. Concerns raised by the woman about the site near Ramsgate include “the routine prolongation of detention beyond statutory time limits; failure to adhere to essential safeguarding measures for children; women and children sleeping alongside adult men to whom they are unrelated; inadequate or nonexistent access to legal advice for those detained; and exposure to infectious diseases due to overcrowding and poor sanitation,” the charity said. Four parliamentary committee chairs increased pressure on the home secretary to explain how the government would get a grip on the situation at the Kent facility and the migrant crisis in general. In a joint letter to Braverman, the chairs of the home affairs committee, the justice committee, the joint committee on human rights and the women and equalities committee expressed their deep concerns over the dire conditions at Manston, asking what would be done to address the current situation and avoid overcrowding in future. Earlier on Thursday, Graham Stuart, a government minister, conceded the site was not operating legally and that “none of us are comfortable with it”. But he sought to blame an “unacceptable surge” in small boat crossings for the problem, adding that the “system is struggling to cope”. The home secretary shied away from press questions during her visit to Western Jet Foil in Dover – the scene of a petrol bomb attack on Sunday – amid concerns of rising far-right activity.