Reform UK would have held just 15% of the donations it received last year if a proposed £100,000 cap on political donations had been in force, according to analysis shared with the Guardian.

The analysis by Friends of the Earth using Electoral Commission data highlights the party’s reliance on a handful of wealthy backers in advance of a showdown over political funding.

It registered donations between April 2025 and March 2026 and assumed union affiliation payments would be exempt from the cap, in line with recommendations made by the Phillips review into party funding.

The findings suggest Reform UK would have raised just £4.1m between April 2025 and March 2026, instead of the £26.7m it actually received, if a £100,000 annual donation limit applied.

Reform’s registered average donation last year was £137,496, almost six times higher than Labour’s £23,406 or the Conservatives’ £23,173 and 30 times higher than the Liberal Democrats’ average donation of £4,496.

By comparison, Labour would have retained about three-quarters of its registered donations under the cap, raising £8.1m rather than £10.8m. The Tories would have kept just over half of their donations, taking £8.3m instead of £15.5m. The Lib Dems would have held on to about 90% of theirs, taking £5.2m instead of £5.8m, and the Greens would have been unaffected with their £468k in donations.

Reform would no longer be Britain’s best-funded political party under the cap, according to the analysis. Labour, the Conservatives and the Lib Dems all would have raised more over the same period.

The figures come before Tuesday’s report stage of the representation of the people bill, when Stella Creasy, the Labour MP for Walthamstow, is expected to table an amendment that would introduce a £100,000 cap on political donations from permitted donors. The proposal comes amid continuing debate over the influence of wealthy donors in British politics.

The analysis also found Reform received £20.4m from donors who each contributed at least £1m during the period analysed, compared with £3.1m for the Conservatives and £2.6m for Labour. The campaign group highlights that Labour’s total donation count is made up largely of trade union affiliation payments, which it argues should be treated differently because they are funded through the political levies of hundreds of thousands of individual union members instead of single wealthy backers.

The campaign group said two billionaire donors – Christopher Harborne, Britain’s sixth richest person, and Ben Delo – accounted for 71% of Reform’s registered donation income over the last year.

Harborne, a British billionaire based in Thailand, has donated £15m to the party. Harbone told the Telegraph in April he believed he could challenge any donation cap in court and did not rule out returning to the UK if changes to the law prevented him from donating while overseas.

A Reform spokesperson said: “Reform UK complies fully with UK electoral law and the suggestion that legitimate donations from successful individuals are somehow less valid than funding from trade unions, for example, is absurd.

“Meanwhile, a £100,000 cap on donations would do nothing to improve democracy. It would simply restrict political participation while entrenching the established parties, which benefit from longstanding institutional funding networks.”

One of biggest Britain’s unions, the GMB, told its affiliated Labour MPs not to vote for the cap,. Party whips are understood to have been calling MPs urging them to follow the union’s warning, causing some to drop their support for the cap.

Asad Rehman, the chief executive of Friends of the Earth, said: “Democracy shouldn’t be for sale. When political parties rely on money from fossil fuel interests and other major polluters, it undermines trust that decisions are being made in the public interest.

“The stakes couldn’t be higher when those same parties are calling to scrap climate action, expand oil and gas drilling, and weaken environmental protections.

“A meaningful cap on political donations would help level the playing field, making parties more accountable to the people they represent, not the biggest chequebooks. Building a fairer, greener future depends on people having confidence that our democracy works for everyone, not just the super-rich.”

A Ministry of Housing, Communities and Local Government spokesperson said: “We are already taking robust action to tackle foreign interference in our democracy through our landmark representation of the people bill, including capping donations from overseas electors and banning donations made via crypto currency.”