The Wedgwood Museum is one of four museums up for the £100,000 prize
An investigation has begun into alleged voting irregularities in a poll for the £100,000 Art Fund Prize.
The People's Choice poll on the website of the Guardian newspaper was closed early after "potential irregularities" were uncovered.
The winner of the poll will count as one vote at the final judges' meeting.
Glasgow's Kelvingrove Art Gallery and Museum; the Wedgwood Museum; the Ruthin Craft Centre and Orleans House Gallery are in the running.
The annual Art Fund Prize - the UK's largest single arts prize - honours the most imaginative and original museum or gallery of the year. The winner is due to be announced on 18 June at the Royal Institute of British Architects in London.
The seven-strong judging panel includes film-maker Lord Puttnam and Turner prize winner Grayson Perry. This was the first year the public could have their say.
The Guardian said in a statement: "Since it appears that there may be some potential irregularities with the online voting for the Art Fund Prize, we have regretfully closed the poll early while we undertake a full investigation into the matter."
On 5 June, the Wedgwood Museum in Stoke-on-Trent was ahead in the poll with almost half the vote, while the Orleans Gallery in Twickenham and Ruthin Craft Centre in Denbighshire, were also doing well.
Kelvingrove, however, appeared to be lagging behind with 10% of the vote.
While Kelvingrove had rallied in recent days, Wedgwood had regained the lead by the time voting closed around 12 hours early. The poll had been due to close at midnight on Thursday.
Gaye Blake Roberts, director of the Wedgwood Museum Trust, said she did not know what sparked the investigation.
"We would be concerned if any zealous supporters had decided to cast extra votes. It is not something we would support and I think all the museums would feel the way we do," she said.
"We would not want a cloud over the results of what is a very prestigious award."
Rachel Tranter, head of arts at Richmond council - which own Orleans House Gallery - said: "We were informed this morning by the Art Fund Prize that the People's Choice poll was closed early over irregular fluctuations in the voting patterns in the 24 hours before voting was closed.
"We understand that the Guardian is working very hard to establish who the true winner of the poll is and we are confident they will achieve this before the judges make their final decision."
A spokesman for the Kelvingrove Museum and Art Gallery said he would not comment while the nature of the allegations were unclear, as did Philip Hughes, director of the Ruthin Craft Centre.