Impositions were introduced purposely and formally in the reign of James I. Impositions were a new source of royal income and were introduced in an effort to keep up with royal expenditure and also in an effort to reduce royal debt - both seemingly very difficult tasks for the likes of Robert Cecil and Lionel Cranfield, Lord Treasurer from 1621 to 1624. Impositions were duties on trade, additional to Customs duties, and were granted to James for life by his first Parliament.
In 1606 impositions received a full legal standing from the Court of Exchequer which stated in its judgment that all imports and exports had to go through a royal port and that "they (the ports) are the gates of the king, and he hath absolute power by them to include or exclude whom he shall please."
Cecil and the then Lord Treasurer, Dorset (Sir Richard Sackville), wanted to use the judgment as soon as was possible to raise much needed revenue. However, it was felt that a cautious approach would be best suited to their introduction as finance and where royal revenue came from was a contentious issue. Those who advised James believed that raising revenue via loans was a better short term solution to any financial issue James had as it did not smack of yet another tax increase in whatever sector of the economy that was affected. Rebellion in Ireland and the money needed to quell it meant that the government needed a more long term financial solution and that came from impositions. Cecil consulted with merchants on what commodities should have impositions put on them. Though merchants had not been included in discussions that led to the introduction of impositions, the inclusion of them at this stage did give them a modicum of ownership as to what was included - spices, silk etc.
The new Imposition was officially introduced in 1608. It brought in about £70,000 a year. Though this was a good sum for the royal coffers, it did not do a great deal to reduce the overall royal debt. As a result of this, Cecil did something that he was reluctant to do - sold crown land.