A total of 69% of child and adolescent admissions for severe mental health issues in 2016/17 were classed as 'out of area', according to data obtained from hospitals by the BMA.
Figures obtained under the Freedom of Information Act show that the proportion of children admitted to hospital out of their area rose 12 percentage points in 2016/17 compared with the previous year.
The BMA warned that the figures - published to coincide with Mental Health Awareness Week - showed worsening access to specialist beds.
The data show that south-west England saw a 106% rise in patients treated out of their local area - the highest rise - while out-of-area admissions rose 92% in Yorkshire and Humber.
GPC deputy chair Dr Richard Vautrey said: 'These are children who will be looked after solely in specialist centres so GPs won't be involved directly with their care during their admission. However they will often be aware of the patient and are also likely to be supporting other members of the family who are often impacted by the distress caused by this situation.
'Building a relationship between the GP and a specialist team well outside their normal practice area can also be difficult. This is yet another sign of how much pressure mental health services are under and why we need politicians of all parties to make this a priority to resolve.'
BMA community care committee chair Dr Gary Wannan said: 'These figures show, alarmingly, that well over half of patients are being placed out of area at a time when they are at their most vulnerable.
'NHS England has promised action on this and we need to have a very clear plan about how it will roll this out to improve this situation for young people and their families. It can be an incredible wrench for children to leave their homes and being based far away is not going to help a young person in crisis.'
A manifesto published by the BMA ahead of the 8 June general election urged all political parties to deliver on the existing commitment to ensure parity of esteem between physical and mental health services.
An NHS England spokesman said: 'Since the definition of out-of-area placements has been toughened up, year on year comparisons will not be accurate. But looking out over the next two years, we are committed to ensuring that children and young people receive care closer to home so 49,000 extra children and young people will get the care they need and we are funding 150-180 new CAMHS Tier 4 specialist inpatient beds in underserved parts of the country, reducing the need for patients having to travel long distances to get the right care.'