The Electoral Commission has dismissed allegations made by Tory MP Priti Patel about the spending of pro-EU groups during the 2016 Brexit referendum.

The watchdog said there was no evidence that laws on campaign spending had been breached.

Ms Patel, a prominent Leave supporter, had suggested that the cost of three videos from the main Remain group, Britain Stronger in Europe (BSIE) should have been declared as "joint spending" with other groups.

The alleged collaboration could have resulted in the lead group exceeding its £7m spending limit, she said.

The former international development secretary, who was forced to resign last year over undeclared meetings with the Israeli government, had called on the Commission to either investigate the Remain campaign or drop its inquiry into Vote Leave for similar spending breaches.

Having investigated the allegations, the watchdog said there was "not sufficient grounds" to suspect rules had been broken.

The Commission said a company called DDB, which also operated under the name Adam and Eve, had been involved in making the videos but had correctly reported its spending.

"In this matter, there is not sufficient grounds to suspect that BSIE breached the joint spending rules," it said.

However, the Commission said information provided by Ms Patel had led it to open an investigation into another campaign group, called Wake Up and Vote, regarding potentially undeclared joint spending with DDB.

The videos in question included one titled "Don't F*** My Future" that featured Keira Knightley and cost more than £76,000 to produce.

Ms Patel also claimed a number of campaigners with links to BSIE had been late in registering payments, but the Electoral Commission said: "The Commission has not been provided with, or found evidence for an investigation to be opened."

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The watchdog also hit back at Ms Patel's claims that its investigations were politically motivated.

In her response to the MP, the Commission's head of regulation, Louise Edwards, said: "Finally, I must express my disappointment at your continued suggestion that the Commission’s impartiality has been compromised in respect of how we regulate EU referendum campaigners....Our investigations cover parties from across the political spectrum, and campaigners from both sides of the EU referendum."

Responding to Ms Patel's suggestion that the watchdog should investigate groups on both sides of the Brexit debate or neither, she added: "I appreciate your support for the Commission’s role and independence and believe the best way for us to maintain our independence is to continue our evidence-led approach.

"It would be wrong for us to take decisions on the basis that we must balance action in respect of ‘leave’ campaigners with action in respect of ‘remain’ campaigners. That would undermine our neutrality, rather than ensure it."

The Electoral Commission also said it had decided not to open an investigation into claims the DUP failed to declare joint spending with other referendum campaigners.

The allegations were made in a BBC Northern Ireland programme called "Spotlight - Brexit, Dark Money and the DUP". The watchdog said it had asked BBC Northern Ireland for evidence but was told there was no further "significant information" available.