The Ministry of Justice could use a prison ship to house inmates, it has emerged.
The country's prison population stood at 81,133 this week, less than 1,000 places short of an overall capacity of 82,104.
A Ministry of Justice spokesman said: "We are looking at all the options for increasing prison capacity including acquiring a ship or barge that could be converted into use as a prison. In any such acquisition we would need to consider the value for money comparison with land-based building programmes."
The admission that ships could be used to ease inmate overcrowding was criticised by prison reformers.
The Director of the Howard League for Penal Reform, Frances Crook, said the only way to solve the problem was to send fewer people to jail.
She said: "A Spanish Armada of prison ships isn't going to solve the prisons crisis.
"The government has been warned for years, by the Howard League and others, that the consequences of their sentencing policies would lead to mass overcrowding, a rise in suicides and strikes by prison staff. And so it has come to pass.
"Community sentences have been proven to reduce reoffending by up to 22%. The government needs to urgently put resources into strengthening community sentencing and raising public confidence in their effectiveness at cutting crime."
Her views were echoed by the Director of the Prison Reform Trust, Juliet Lyon.
She said: "It is tragic that the Government is scrabbling around for yet more places for offenders when to cut crime it should be working flat out to develop drug treatments, better mental health care and more effective community punishments."
