A woman missing with her three-year-old son has “gone underground”, lawyers say.

Ellie Yarrow-Sanders has not used her bank accounts or mobile since disappearing with Olly Sheridan six months ago, according to solicitors representing the youngster’s father, Patrick Sheridan.

Staff at law firm Irwin Mitchell on Thursday renewed appeals for help finding Olly and his 26-year-old mother and said the situation was becoming “increasingly serious”.

Olly is at the centre of a family court dispute between his estranged parents.

Olly Sheridan missing
Three-year-old Olly Sheridan has gone missing with his mother Ellie Yarrow-Sanders (Patrick Sheridan)

Miss Yarrow-Sanders disappeared with Olly in July a few weeks before she was due to give evidence in a “critical” family court hearing, lawyers say.

A judge raised the alarm in December and urged anyone with information to come forward.

Mr Justice Williams, who had overseen a hearing in the Family Division of the High Court in London, said he was gravely concerned.

Family court hearings have been staged in private and journalists had been barred from identifying any involved family members, who come from the Basildon area of Essex.

But Mr Justice Williams said he was lifting the identity bar in the hope that publicity would help find Olly and Miss Yarrow-Sanders.

He said they could be named and photographs published.

Olly Sheridan missing
Patrick Sheridan with his three-year-old son Olly (Family Handout)

Essex Police are also searching and have urged anyone with information to get in touch.

“Ellie has gone underground since abducting Olly,” said an Irwin Mitchell spokeswoman on Thursday.

“She has not used her bank accounts or mobile phone since she and Olly disappeared in July.”

The spokeswoman said Miss Yarrow-Sanders’ mother had travelled to Huddersfield, West Yorkshire, at around the time Olly vanished.

She said Miss Yarrow-Sanders’ and Olly might also have stayed in the Huddersfield area.

Lawyer Jenna Lucas added: “The situation for Olly Sheridan is increasingly serious.”

Mr Sheridan, 45, said Christmas had been “unbearable”.