The Militia Ordinance was a piece of legislation passed by the Long Parliament of England in March 1642, which was a major step towards the Civil War between the King and Parliament of England. Previously the King had the sole right to appoint the Lord Lieutenants, who were in charge of the county militias (also known as the trained bands). These militias were the only land forces available in peacetime, because England had no regular standing army. As relations between King and Parliament deteriorated, control of the militia became a very controversial issue. Parliament did not trust Charles and tried to deny him control of any military forces which he might use against them. Charles I attempted to arrest five members of the Commons and one Lord in January 1642, but the episode resulted in the five members being hailed heroes and Charles' departure from London. On 5 March 1642 the anti-royalist majority in the House of Lords passed an Ordinance appointing their choice of Lieutenants, although a royalist minority protested. The House of Commons agreed to the Ordinance the same day, but according to English custom no legislation could become law until it received the royal assent. The King refused to give his assent to the bill, but on 15 March 1642 Parliament declared that "the People are bound by the Ordinance for the Militia, though it has not received the Royal Assent". This was the first time that Parliament had ever put a law into effect without royal assent. Such an unprecedented assertion of Parliamentary sovereignty made war far more likely.
The Militia Ordinance and the King's Commissions of Array were statements of intent and tests of loyalty but had little practical impact on the raising of armies. The main Parliamentarian army commanded by the Earl of Essex was formed entirely with regular regiments raised from volunteers who enlisted in the summer of 1642. Local trained bands often had divided loyalties or refused to serve outside their own counties. The biggest exception was the London Militia, which was placed under the control of the London Militia Committee by the Militia Ordinance. London was by far the most populous city in England and had a large and well trained militia. The London trained bands marched out to link up with Essex's army at Turnham Green in November 1642, blocking a royalist advance on London. In subsequent years London trained bands regiments were used to reinforce the armies of the Earl of Essex and Sir William Waller, and the Essex trained bands took part in the siege of Reading in 1643. The Long Parliament continued to pass and enforce Ordinances without the royal assent throughout the 1640s. Most of these were declared void after the Restoration, but Charles II continued the excise duty, which was originally brought in by an Ordinance of Parliament in 1643.