Royal Navy warship will leave Portsmouth today to patrol English Channel and tackle Dover migrant crossings

Portsmouth-based patrol ship HMS Mersey is to be sent to the English Channel as part of a fresh effort to block illegal migrants illegally entering Britain.

The news has not yet been confirmed by the Ministry of Defence or the home office, with official sources only saying that the nation's military '˜stand ready to provide additional capacity'. But Portsmouth sources say the ship will leave the city at 3pm today.

It's reported the home secretary Sajid Javid last night called in the military as he tried to get a grip on the Channel migrant crisis.

The Daily Mail claimed Mersey would be supported by the Royal Air Force, which will provide aerial surveillance.

The move is a U-turn from the home secretary's previous position, in which he refused an offer of military assistance from defence secretary Gavin Williamson.

Mr Javid, who is tipped as one of those in the running to success Theresa May as prime minister, has come under fire for his response to the situation.

At least 139 migrants were caught crossing from France to Britain over Christmas, and 239 have reached the UK since November.

The situation prompted the home secretary to cut short a family safari holiday in South Africa and fly back to Whitehall on Monday.

He later ordered two border force cutters from the Mediterranean to return to the UK to bolster the nation's security.

The MoD is in direct contact with the home office to establish what defence assets could be used.

It's understood the plan could involve HMS Mersey being stationed off the Kent coast.

It will provide cover while the border force patrol boats return from the Mediterranean, which could take as long as a fortnight.

Royal Navy sailors will also be sent to help man the civilian vessels when they eventually get back so they can operate around the clock.

The RAF will provide aerial surveillance to track boats trying to make the crossing.

A spokesman for the MoD said: '˜Our armed forces stand ready to provide additional capacity and expertise to assist the Home Office with the response to migrant crossings.

'˜Royal Navy ships continue to conduct patrols to protect the integrity of UK territorial waters.'

HMS Mersey, and her two sister ships Tyne and Severn, had been earmarked to be decommissioned.

However, last year the government announced plans to save three ageing patrol ships and use them to bolster Britain's borders.

The news comes just days of city leaders hit out at the government's reaction to the migrant crisis.

As previously reported, Councillor Donna Jones, former leader of Portsmouth City Council, said it would be unlikely to deploy the navy and described initial claims as '˜over-the-top'.