REFERENDUM watchdogs are coming under pressure to block the registration of a string of pro-independence groups amid claims they are being used as a front to boost the Yes campaign's spending power in the run-up to the vote.

Unionists are concerned a long list of pro-independence groups linked to the main Yes Scotland organisation will be recognised as official campaign bodies after the Electoral Commission yesterday approved one, the Scottish Independence Convention, as a so-called "permitted participant".

Under referendum rules, permitted participants are allowed to spend up to £150,000 campaigning in the final 16 weeks before the vote.

The cash does not count towards ­official lead campaign body Yes Scotland's £1.5 million spending limit provided the organisations do not work together or co-ordinate their activity.

Three other pro-Yes groups are either in the process of registering or have signalled their intention to apply to the Electoral Commission.

Pro-UK supporters fear the Electoral Commission will struggle to police the campaign if a large number of pro-Yes groups are given the go-ahead to campaign autonomously.

They warned yesterday that multiple registrations risked undermining strict spending rules, designed to ensure a level playing field in the battle for votes, and create a "free for all" instead.

Tory MSP John Lamont - who has campaigned on behalf of the cross-party pro-UK Better Together group - said: "We cannot have the situation where organisations are being set up simply to break the rules and spend money over and above the limits set for the campaigns.

"The Yes camp cannot credibly claim that they are not working with groups like Business For Scotland.

"The money that is being spent must come off the Yes Scotland total."

He added: "Now that groups are ­registering as permitted participants in the referendum, we have to have confidence that the rules are being followed."

The Scottish Independence Convention is chaired by actress Elaine C Smith, who is also on the board of Yes Scotland. The organisation's website features the Yes Scotland logo and a link to the main campaign.

Other pro-independence groups also have close connections with Yes Scotland. Business for Scotland, which is in the process of applying to be an official campaigner, and Christians for Independence, which has signalled its ­intention to follow suit after receiving a £100,000 donation from bus tycoon Sir Brian Souter, are both promoted on Yes Scotland's website.

They belong to a list of 16 "interest groups" which visitors to the Yes Scotland website are encouraged to join.

Another group, arts-based National Collective, which is also understood to be in the process of applying, has been cited by Yes Scotland chief executive Blair Jenkins as showing "the scale and breadth of our campaign".

Yes Scotland and rival Better Together were last week approved as the official lead organisations in the campaign by the Electoral Commission.

Yes Scotland's application was backed by many of the groups it helps to promote, including Women for Independence, Scots Asians for Yes, Yes LGBT, Farmers for Yes, Labour for Independence and the Scottish Independence Convention.

In its submission to the watchdog, Yes Scotland stressed it was "not an umbrella organisation in the strict sense". However, it confirmed: "We have effectively been representing other campaigners since the campaign has properly begun."

It said it had helped set up a number of groups, including Academics for Yes, Polish for Yes and Crofting for Yes.

Rival Better Together has not allowed its local or interest groups to have their own bank accounts in a bid to comply with Electoral Commission rules. But a source said they would register as separate campaigning organisations if a string of Yes groups are given the go ahead by the watchdog.

It is feared the development would leave the Electoral Commission with an almost impossible task policing the referendum during the 16 weeks leading up to the vote, known as the "regulated period," when spending rules apply.

A spokesman for Yes ­Scotland said: "Yes Scotland adheres strictly to Electoral Commission rules on donations."

An Electoral Commission spokeswoman said: "The rules on campaigning at the Scottish Independence Referendum during the referendum period between May 30 and September 18 are clearly set out in law. It's the responsibility of campaigners to comply with the rules.

"We're already working with those who have registered as permitted participants and other campaigners to help them understand and follow the campaigning rules and published clear guidance for campaigners last December. The Electoral Commission's priority is to ensure that voters have confidence in the referendum outcome and we are ready to deal appropriately with any breach of the rules."

l Scottish Labour are hiring 12 referendum campaign assistants to help organise local party members and activists in the run-up to the vote.