A downhole drilling method for drilling a well in a formation having a formation pressure, comprising: - providing a drill string having a first part and a second part, the second part being arranged closer to a top of the well than the first part, and the first part having a drilling head in a first end and an annular barrier arranged closer to the top of the well than the drilling head, the annular barrier comprising: - a tubular metal part for mounting as part of the first part of the drill string, the tubular metal part having an outer face, - an expandable metal sleeve surrounding the tubular metal part and having an inner face facing the tubular metal part and an outer face facing an inner face of a borehole of the well, each end of the expandable metal sleeve being connected with the tubular metal part, and - an annular space between the inner face of the expandable metal sleeve and the tubular metal part, the expandable metal sleeve being configured to expand, - drilling the borehole by means of the drilling head, - detecting the formation pressure to determine any loss of formation pressure, - stopping the drilling, - dropping a ball into the drill string, - pressurising the drill string until the ball reaches a ball seat arranged opposite or below the annular barrier, - expanding the expandable metal sleeve by further pressurising the drill string until the expandable metal sleeve abuts the inner face of the borehole, - separating the second part of the drill string from the first part, - pulling the second part out of the borehole, and - injecting cement into the borehole above the first part to provide a cement plug above the first part. - providing a drill string having a first part and a second part, the second part being arranged closer to a top of the well than the first part, and the first part having a drilling head in a first end and an annular barrier arranged closer to the top of the well than the drilling head, the annular barrier comprising: - a tubular metal part for mounting as part of the first part of the drill string, the tubular metal part having an outer face, - an expandable metal sleeve surrounding the tubular metal part and having an inner face facing the tubular metal part and an outer face facing an inner face of a borehole of the well, each end of the expandable metal sleeve being connected with the tubular metal part, and - an annular space between the inner face of the expandable metal sleeve and the tubular metal part, the expandable metal sleeve being configured to expand, - a tubular metal part for mounting as part of the first part of the drill string, the tubular metal part having an outer face, - an expandable metal sleeve surrounding the tubular metal part and having an inner face facing the tubular metal part and an outer face facing an inner face of a borehole of the well, each end of the expandable metal sleeve being connected with the tubular metal part, and - an annular space between the inner face of the expandable metal sleeve and the tubular metal part, the expandable metal sleeve being configured to expand, - drilling the borehole by means of the drilling head, - detecting the formation pressure to determine any loss of formation pressure, - stopping the drilling, - dropping a ball into the drill string, - pressurising the drill string until the ball reaches a ball seat arranged opposite or below the annular barrier, - expanding the expandable metal sleeve by further pressurising the drill string until the expandable metal sleeve abuts the inner face of the borehole, - separating the second part of the drill string from the first part, - pulling the second part out of the borehole, and - injecting cement into the borehole above the first part to provide a cement plug above the first part.