The British Horseracing Authority's investigation into the spate of positive tests for morphine is close to a conclusion with contaminated feed the likely cause.

The Queen's 2013 Ascot Gold Cup winner Estimate is one of seven horses known to have tested positive for the painkiller in recent weeks.

BHA chief executive Paul Bittar says the investigation points to feed contamination although the source has yet to be officially confirmed.

"I wouldn't rule out there being no further positives at this stage, but we are very close to concluding our investigation," Bittar told Channel 4 Racing.

"At this stage all our information points towards a feed contamination, as has been reported.

"It is certainly possible there may be one or two more (positives), but we are very near the end of the investigation.

"If we find that contamination is the sole reason, then we have opportunities under the Rules about how we deal with the trainers."