THE watchdog policing the ­independence referendum has warned campaigners to abide by the rules as the battle enters a new phase.

The campaign today begins its "regulated period", when legally binding rules on spending and donations come into force.

The milestone was marked by low-key events staged by the two lead campaign organisations, Better Together and Yes Scotland, which are waiting for the 100-days-to go landmark in 11 days' time to step up their campaigns.

The Electoral Commission, which will monitor campaigning activity in the remaining 16 weeks before polling, has urged all those planning to take part in the referendum battle to stick to the rules.

John McCormick, Electoral Commissioner for Scotland, said: "Campaigners have a vital role to play at any referendum as they set out competing views for voters to choose between.

"At the same time, voters will want to be confident that campaigners are playing by the rules and that there is transparency about how the campaigns are funded.

"We've been working with campaigners to make sure they understand their responsibilities and can comply with the rules and we will be monitoring their ­activities closely."

Under the rules, spending by Better Together and Yes Scotland is capped at £1.5 million between now and polling day.

Political parties have their own cash limits, based on their share of the vote at the last Holyrood election, with the SNP's highest at £1,344,000 followed by Scottish Labour's cap of £834,000. Other registered participants are allowed to spend up to £150,000.

Any un-registered individual or group that spends more than £10,000 will be in breach of the law, and risk a fine. The Electoral Commission will also be able to go to court to obtain a "stop notice" to halt their activities.

As of last night, 14 individuals, groups and political parties had registered with the watchdog, though others are expected to become formal participants.

Among the campaign ­organisations are Business for Scotland and Women for ­Independence, which have links with Yes Scotland, and the pro-UK No Borders group, which has paid for a number of controversial cinema adverts.

Nine of the 14 registered campaigners are pro-Yes.

Registered campaigners must report any donations over £7500, which will be published over the summer. They are also obliged to accept donations only from "permitted sources," for example companies based in the UK.

Campaign groups also face strict rules on working together in a bid to artificially increase their spending limits.

In a statement, the Electoral Commission warned it was ready to investigate "substantive allegations" against campaigners.

The watchdog will also monitor well-known campaigners who have so far not registered.

Last night, in a lecture at ­Glasgow University, Yes ­Scotland chief Blair Jenkins said independence would enable ­Scotland to tackle the widening gap between rich and poor.

He said: "The Yes narrative is that an independent Scotland is more likely to produce a social and economic model that is in line with the values and priorities of the people of Scotland. This would be a fairer, more progressive, peaceful and reforming Scotland."

Alistair Darling, the leader of Better Together, yesterday joined No campaign activists in Glasgow to highlight "1400 reasons" for staying in the UK - a reference to the UK Government's claim that Scots would be £1400 worse off under independence.

l Lord David Steel, Holyrood's first Presiding Officer, will join follow Liberal Democrat peer Baroness Shirley Williams in Glasgow today to make "the ­positive case" for the Union.