No decision is imminent to authorise the use of water cannon on the streets of Britain despite the fact that the first of three German vehicles bought by the mayor of London has already been delivered, the home secretary has said.

Boris Johnson decided not to wait for Theresa May's authorisation before spending £218,000 on the three second-hand Wasserwerfer 9000 water cannon.

The mayor has said he wanted them to be available for the Metropolitan police to use on the streets of London this summer but that now looks highly unlikely.

May told MPs that she could not yet give a timeframe for her required decision on whether to authorise the use of water cannon, adding that there were scientific and medical issues to be resolved.

When Keith Vaz, the chairman of the Commons home affairs select committee, questioned her over the state of play on water cannon, he claimed that they had arrived but had been "hidden by Boris – nobody knows where they are".

The machines arrived in Britain on 30 June and have been fitted out for British conditions, including the installation of CCTV cameras.

May told the MPs on Monday there were still a number of medical and scientific issues, including testing, to be clarified before she could take a decision on whether to authorise their use on mainland Britain. They have been in use in Northern Ireland for a number of years.

The home secretary conceded that "there were several operational issues" with water cannon and opinion among senior police officers varied sharply as to their usefulness.

The London mayor has said the machines were bought early because they were available cheaply from the German federal police.