THE SNP is at the centre of a lobbying row after ministers backed a third runway at Heathrow just days before the airport helps bankroll the party's conference.

The Scottish Government had previously refused to side with either Heathrow or its rival Gatwick in the debate over expanding London’s air capacity.

However Economy Secretary Keith Brown yesterday said Heathrow offered "the best deal for Scotland" in terms of improved connectivity for passengers and up to 16,000 new jobs.

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But opposition parties queried the announcement after it emerged Heathrow had struck a “bespoke commercial arrangement” to be part of this week's SNP conference in Glasgow.

The airport is hosting a “private airport-style lounge” and bar for three days in the Clyde Auditorium, and has bought a full page advert in the conference agenda.

Heathrow also supported the recent Labour, Tory and LibDem conferences, but no other party has formally backed Heathrow, making the SNP the first to give the airport a useful boost.

Heathrow and the SNP refused to reveal how much the airport was paying, but the party has been charging up to £23,000 to sponsor conference events.

LibDem MSP Alex Cole-Hamilton, whose Edinburgh Western constituency includes the capital’s airport, said: “Businesses are perfectly entitled to contact politicians and make their views known and party conferences often are the venue for these meetings.

“But the fact this announcement was made days before Heathrow are due to splash out at SNP conference will raise a few eyebrows.

“People need to know when the SNP took the decision to back the third runway, and that Heathrow’s presence at SNP conference had no bearing on the announcement.”

Read more: SNP condemn 'irresponsible' Tories as anger grows over fears MPs could be sidelined on Brexit

Green MSP John Finnie said: “On one hand the SNP claims to be a party of social justice and climate leadership, but on the other is happy to take substantial sums from airports lobbying for growth. Many of its supporters will be uncomfortable with this kind of corporate cosiness.”

Heathrow and Gatwick both courted the SNP after it became the third largest party at Westminster in 2015, setting up lounges at last year’s SNP conference in Aberdeen.

However only Heathrow is hosting an event at conference this week.

Its involvement is not traditional sponsorship - paying money to puts its name to an existing event - but a bespoke “commercial arrangement” with the SNP.

Mr Brown said the government had engaged extensively with both Gatwick and Heathrow.

He said: “Gatwick’s proposals are also to be commended. However, Heathrow’s plan offers significant job creation, major investment opportunities and, crucially, seeks to address how all of Scotland’s airports benefit from the new runway capacity."

After years of delays, and the Airports Commission backing the £17bn expansion of Heathrow in 2015, the UK government is expected to back a third runway within weeks.

If MPs back the plan, Scottish Government-owned Prestwick Airport could become a logistics hub for it.

Read more: SNP condemn 'irresponsible' Tories as anger grows over fears MPs could be sidelined on Brexit

Heathrow bosses have also promised £10m to support new domestic routes and a £10 per passenger discount from January 2017, reducing airline operating costs from Aberdeen, Edinburgh, Glasgow and Inverness.

In exchange for Scottish Government support, Heathrow will deliver "a significant, long term marketing campaign at Heathrow to promote all that Scotland has to offer".

Richard Dixon, director of Friends of the Earth Scotland, said the SNP’s support for more flights jarred with claim to be tackling climate change: “They are backing the expansion of one of our biggest airports, which will result in 70 per cent more traffic and emissions. If we’re concerned about climate change we can’t possibly expand aviation even more.”

A Gatwick spokesman said the Scottish Government had backed an option "making it more dependent on London and the South East", adding: "Some of the suggested benefits to Scotland are unlikely to bear serious scrutiny, which makes the decision all the more curious."

A Heathrow spokeswoman said: “Heathrow is the hub airport for all of Britain and the number one destination for Scottish passengers. We also have over 100 employees in our Business Support Centre in Glasgow. SNP Conference is a great opportunity for us to meet Scottish elected representatives and discuss issues relevant to passengers and employees.”

An SNP spokesman said: “This sort of frenzied reaction from the Lib Dems would be laughable were it not so contrived, predictable and depressing. The expansion plans from Heathrow offer the most considerable benefits to Scotland - £200m in construction spending, 16,000 new jobs and a huge boost to connectivity including £10m investment in new routes.”

The Herald revealed last month that the SNP hopes to boost its coffers by £160,000 this week via 19 sponsorship deals and a £200-a-plate dinner, with firms asked £15,500 to put their names on lanyards, £12,500 for delegate bags and £2,000 to sponsor a creche.