The study showed that the Conservatives would win 305 seats, fewer than won by David Cameron and shy of the 326 needed to pass laws with a majority.
It follows a string of concerning polls for the Prime Minister, the most recent of which from ICM showed her party's lead further narrowing by one point.
When Ms May called the election, her party was on course for a landslide win and had poll leads of around 20 points.
On Saturday, YouGov said its model suggested the Conservatives were on course to win 308 seats but the number had fallen by Monday.
The opposition Labour Party is likely to win 268 seats, YouGov's model showed on Monday, up from 261 on Saturday.
Another model, produced by Lord Ashcroft Polls, last week predicted the Conservatives were on course for a majority.
Last week ICM published a poll showing that the Conservatives had a lead of 12 points, but this morning it had narrowed.
The most recent survey showed support for the Conservatives at 45 per cent, unchanged from last week, but Labour was up one point to 34 per cent.
The Conservatives' lead as measured by ICM is wider than in several opinion polls conducted by other firms.
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