Admiral-Lord Mountevans rules were a set of professional wrestling rules mainly used in the United Kingdom in the second half of the 20th century. Introduced in 1947, they were named after Edward Evans, 1st Baron Mountevans, who ordered a unified set of rules written up for professional wrestling in the British Isles to re-legitimise professional wrestling in the eyes of the public and the press. Professional wrestling in the United Kingdom had become discredited before World War II due to the prominence of the preceding "All In" style, which came to emphasise an "anything goes" style of presentation. Professional wrestling promoters in the United Kingdom rallied behind the Mountevans rules and radically altered how professional wrestling was presented in the United Kingdom thereafter. The style of wrestling under the Mountevans rules was advertised by promoters as Modern Freestyle Wrestling.

Mountevans rules would remain the predominant style of professional wrestling in the United Kingdom until the 1990s, when they were increasingly abandoned in favour of a style and presentation in line with American professional wrestling. Some UK promotions continue to use either the full rules and/or hybrid systems for all or selected matches. Modern UK promotions using Mountevans rules are sometimes referred to as World of Sport Style promotions.

Professional wrestling in the United Kingdom, under the All In rules, had been popular in the 1930s. However, as a result of a shift in focus away from technical skill and towards violence including usage of weapons, it was banned towards the end of the decade by the London County Council.

After a failed attempt to relaunch the sport with a show at Harringay Arena, Middlesex in 1947 was condemned by journalists as being "fake", Admiral-Lord Mountevans along with radio personality Commander Archibald Bruce Campbell, Maurice Webb MP and Norman Morell, an amateur wrestling champion and professional wrestling promoter, formed a committee to formalise professional wrestling in the United Kingdom and write up a set of unified rules.

Once the rules were written, they were almost universally accepted with 95% of promotions in the United Kingdom adopting them. In 1952, the rules were adopted by the newly formed Joint Promotions, officially as a way to uphold the committee's ideology but was also considered to be a way for Joint Promotions to effectively control British wrestling. In practice, independent promotions at the time adhered to the rule system in all but name and most major promoters in mainland western Europe adopted systems broadly modelled on the Mountevans Rules (although French wrestling had largely abandoned rounds by the time it arrived on French TV in the mid 1950s).

In London, the new rules led to the ban on professional wrestling being lifted however a by-law authorised by the Home Office and the Metropolitan Police kept female wrestling banned within the Greater London area until 1987.

Over the course of the 1990s, matches without rounds became increasingly prevalent. In the 21st century, Premier Promotions implements the full Mountevans Rules, with other old school promotions (such as All Star Wrestling and Rumble Wrestling) occasionally promoting high-profile matches (such as certain title bouts) under the full rules but usually adopting hybrid rules mixing elements of the Mountevans system with American rules more familiar to modern fans. Some "New School" (Americanised) promotions also run "World of Sport Rules" matches as a gimmick match sometimes as parody. In 2001, WWE also parodied the Mountevans Rules with its "Duchess of Queensbury Rules match" at Backlash. In 2020, WWE's NXT UK brand created a new secondary championship called the NXT UK Heritage Cup. The matches for the NXT UK Heritage Cup are held in British Round Rules, which adopted parts of the Mountevans Rules.

The rules set out seven weight divisions with maximum weight limits and called for champions to be crowned of each limit : Lightweight (154 lb or ~70 kg), Welterweight (165 lb or ~75 kg), Middleweight (176 lb or ~80 kg), Heavy Middleweight (187 lb or ~85 kg), Light Heavyweight (198 lb or ~90 kg), Mid-Heavyweight (209 lb or ~95 kg) and Heavyweight for all weights above 210 lb (95 kg). In the early days, the champions of these weight divisions were also known as Mountevans Champions as the Championships also included Mountevans in their official names.