Royal Navy vessel HMS Mersey has been deployed to patrol the Straits of Dover amid concern about migrants crossing the English Channel.
The move follows a request from Home Secretary Sajid Javid for the Navy's support in dealing with the situation.
Sending HMS Mersey to patrol the Channel follows Mr Javid earlier this week announcing the redeployment of two Border Force cutters from the Mediterranean.
Defence Secretary Gavin Williamson said: "I can confirm that HMS Mersey will deploy to the Dover Straits to assist the UK Border Force and French authorities with their response to migrant crossings.
"The professionalism of the Royal Navy means the crew have been able to immediately divert from routine operations to help prevent migrants from making the dangerous journey across the Channel."
Mr Javid said: "My focus continues to be on protecting the UK border and preventing loss of life in the Channel.
"For these reasons, the Government has decided to deploy a navy vessel, HMS Mersey, to support our existing efforts.
"This will be an interim measure while the two Border Force cutters I have redeployed from abroad make their way back to UK waters.
"It is vital that we are working on all fronts to tackle this ongoing situation and I am grateful to Border Force and other agencies for the tireless work they have done in response to this activity."
HMS Mersey, which left Portsmouth on Thursday afternoon, is one of four Royal Navy river class patrol vessels.
It launched in June 2013, has a crew of around 45 sailors and a range of 7,800 sea miles, according to the Royal Navy website.
The cost of the offshore patrol vessel's deployment will be met by the Home Office, a spokesperson said.